feat: Adding OpenAI Prompts API (#3319)

# What does this PR do?
This PR adds support for OpenAI Prompts API.

Note, OpenAI does not explicitly expose the Prompts API but instead
makes it available in the Responses API and in the [Prompts
Dashboard](https://platform.openai.com/docs/guides/prompting#create-a-prompt).

I have added the following APIs:
- CREATE
- GET
- LIST
- UPDATE
- Set Default Version

The Set Default Version API is made available only in the Prompts
Dashboard and configures which prompt version is returned in the GET
(the latest version is the default).

Overall, the expected functionality in Responses will look like this:

```python
from openai import OpenAI
client = OpenAI()

response = client.responses.create(
  prompt={
    "id": "pmpt_68b0c29740048196bd3a6e6ac3c4d0e20ed9a13f0d15bf5e",
    "version": "2",
    "variables": {
        "city": "San Francisco",
        "age": 30,
    }
  }
)
```

### Resolves https://github.com/llamastack/llama-stack/issues/3276


## Test Plan
Unit tests added. Integration tests can be added after client
generation.

## Next Steps
1. Update Responses API to support Prompt API
2. I'll enhance the UI to implement the Prompt Dashboard. 
3. Add cache for lower latency

---------

Signed-off-by: Francisco Javier Arceo <farceo@redhat.com>

llama_stack/core/prompts/__init__.py

@@ -0,0 +1,5 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.

llama_stack/core/prompts/prompts.py

@@ -0,0 +1,233 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+import json
+from typing import Any
+
+from pydantic import BaseModel
+
+from llama_stack.apis.prompts import ListPromptsResponse, Prompt, Prompts
+from llama_stack.core.datatypes import StackRunConfig
+from llama_stack.core.utils.config_dirs import DISTRIBS_BASE_DIR
+from llama_stack.providers.utils.kvstore import KVStore, kvstore_impl
+from llama_stack.providers.utils.kvstore.config import SqliteKVStoreConfig
+
+
+class PromptServiceConfig(BaseModel):
+    """Configuration for the built-in prompt service.
+
+    :param run_config: Stack run configuration containing distribution info
+    """
+
+    run_config: StackRunConfig
+
+
+async def get_provider_impl(config: PromptServiceConfig, deps: dict[Any, Any]):
+    """Get the prompt service implementation."""
+    impl = PromptServiceImpl(config, deps)
+    await impl.initialize()
+    return impl
+
+
+class PromptServiceImpl(Prompts):
+    """Built-in prompt service implementation using KVStore."""
+
+    def __init__(self, config: PromptServiceConfig, deps: dict[Any, Any]):
+        self.config = config
+        self.deps = deps
+        self.kvstore: KVStore
+
+    async def initialize(self) -> None:
+        kvstore_config = SqliteKVStoreConfig(
+            db_path=(DISTRIBS_BASE_DIR / self.config.run_config.image_name / "prompts.db").as_posix()
+        )
+        self.kvstore = await kvstore_impl(kvstore_config)
+
+    def _get_default_key(self, prompt_id: str) -> str:
+        """Get the KVStore key that stores the default version number."""
+        return f"prompts:v1:{prompt_id}:default"
+
+    async def _get_prompt_key(self, prompt_id: str, version: int | None = None) -> str:
+        """Get the KVStore key for prompt data, returning default version if applicable."""
+        if version:
+            return self._get_version_key(prompt_id, str(version))
+
+        default_key = self._get_default_key(prompt_id)
+        resolved_version = await self.kvstore.get(default_key)
+        if resolved_version is None:
+            raise ValueError(f"Prompt {prompt_id}:default not found")
+        return self._get_version_key(prompt_id, resolved_version)
+
+    def _get_version_key(self, prompt_id: str, version: str) -> str:
+        """Get the KVStore key for a specific prompt version."""
+        return f"prompts:v1:{prompt_id}:{version}"
+
+    def _get_list_key_prefix(self) -> str:
+        """Get the key prefix for listing prompts."""
+        return "prompts:v1:"
+
+    def _serialize_prompt(self, prompt: Prompt) -> str:
+        """Serialize a prompt to JSON string for storage."""
+        return json.dumps(
+            {
+                "prompt_id": prompt.prompt_id,
+                "prompt": prompt.prompt,
+                "version": prompt.version,
+                "variables": prompt.variables or [],
+                "is_default": prompt.is_default,
+            }
+        )
+
+    def _deserialize_prompt(self, data: str) -> Prompt:
+        """Deserialize a prompt from JSON string."""
+        obj = json.loads(data)
+        return Prompt(
+            prompt_id=obj["prompt_id"],
+            prompt=obj["prompt"],
+            version=obj["version"],
+            variables=obj.get("variables", []),
+            is_default=obj.get("is_default", False),
+        )
+
+    async def list_prompts(self) -> ListPromptsResponse:
+        """List all prompts (default versions only)."""
+        prefix = self._get_list_key_prefix()
+        keys = await self.kvstore.keys_in_range(prefix, prefix + "\xff")
+
+        prompts = []
+        for key in keys:
+            if key.endswith(":default"):
+                try:
+                    default_version = await self.kvstore.get(key)
+                    if default_version:
+                        prompt_id = key.replace(prefix, "").replace(":default", "")
+                        version_key = self._get_version_key(prompt_id, default_version)
+                        data = await self.kvstore.get(version_key)
+                        if data:
+                            prompt = self._deserialize_prompt(data)
+                            prompts.append(prompt)
+                except (json.JSONDecodeError, KeyError):
+                    continue
+
+        prompts.sort(key=lambda p: p.prompt_id or "", reverse=True)
+        return ListPromptsResponse(data=prompts)
+
+    async def get_prompt(self, prompt_id: str, version: int | None = None) -> Prompt:
+        """Get a prompt by its identifier and optional version."""
+        key = await self._get_prompt_key(prompt_id, version)
+        data = await self.kvstore.get(key)
+        if data is None:
+            raise ValueError(f"Prompt {prompt_id}:{version if version else 'default'} not found")
+        return self._deserialize_prompt(data)
+
+    async def create_prompt(
+        self,
+        prompt: str,
+        variables: list[str] | None = None,
+    ) -> Prompt:
+        """Create a new prompt."""
+        if variables is None:
+            variables = []
+
+        prompt_obj = Prompt(
+            prompt_id=Prompt.generate_prompt_id(),
+            prompt=prompt,
+            version=1,
+            variables=variables,
+        )
+
+        version_key = self._get_version_key(prompt_obj.prompt_id, str(prompt_obj.version))
+        data = self._serialize_prompt(prompt_obj)
+        await self.kvstore.set(version_key, data)
+
+        default_key = self._get_default_key(prompt_obj.prompt_id)
+        await self.kvstore.set(default_key, str(prompt_obj.version))
+
+        return prompt_obj
+
+    async def update_prompt(
+        self,
+        prompt_id: str,
+        prompt: str,
+        version: int,
+        variables: list[str] | None = None,
+        set_as_default: bool = True,
+    ) -> Prompt:
+        """Update an existing prompt (increments version)."""
+        if version < 1:
+            raise ValueError("Version must be >= 1")
+        if variables is None:
+            variables = []
+
+        prompt_versions = await self.list_prompt_versions(prompt_id)
+        latest_prompt = max(prompt_versions.data, key=lambda x: int(x.version))
+
+        if version and latest_prompt.version != version:
+            raise ValueError(
+                f"'{version}' is not the latest prompt version for prompt_id='{prompt_id}'. Use the latest version '{latest_prompt.version}' in request."
+            )
+
+        current_version = latest_prompt.version if version is None else version
+        new_version = current_version + 1
+
+        updated_prompt = Prompt(prompt_id=prompt_id, prompt=prompt, version=new_version, variables=variables)
+
+        version_key = self._get_version_key(prompt_id, str(new_version))
+        data = self._serialize_prompt(updated_prompt)
+        await self.kvstore.set(version_key, data)
+
+        if set_as_default:
+            await self.set_default_version(prompt_id, new_version)
+
+        return updated_prompt
+
+    async def delete_prompt(self, prompt_id: str) -> None:
+        """Delete a prompt and all its versions."""
+        await self.get_prompt(prompt_id)
+
+        prefix = f"prompts:v1:{prompt_id}:"
+        keys = await self.kvstore.keys_in_range(prefix, prefix + "\xff")
+
+        for key in keys:
+            await self.kvstore.delete(key)
+
+    async def list_prompt_versions(self, prompt_id: str) -> ListPromptsResponse:
+        """List all versions of a specific prompt."""
+        prefix = f"prompts:v1:{prompt_id}:"
+        keys = await self.kvstore.keys_in_range(prefix, prefix + "\xff")
+
+        default_version = None
+        prompts = []
+
+        for key in keys:
+            data = await self.kvstore.get(key)
+            if key.endswith(":default"):
+                default_version = data
+            else:
+                if data:
+                    prompt_obj = self._deserialize_prompt(data)
+                    prompts.append(prompt_obj)
+
+        if not prompts:
+            raise ValueError(f"Prompt {prompt_id} not found")
+
+        for prompt in prompts:
+            prompt.is_default = str(prompt.version) == default_version
+
+        prompts.sort(key=lambda x: x.version)
+        return ListPromptsResponse(data=prompts)
+
+    async def set_default_version(self, prompt_id: str, version: int) -> Prompt:
+        """Set which version of a prompt should be the default, If not set. the default is the latest."""
+        version_key = self._get_version_key(prompt_id, str(version))
+        data = await self.kvstore.get(version_key)
+        if data is None:
+            raise ValueError(f"Prompt {prompt_id} version {version} not found")
+
+        default_key = self._get_default_key(prompt_id)
+        await self.kvstore.set(default_key, str(version))
+
+        return self._deserialize_prompt(data)