feat: Add OpenAI Conversations API (#3429)

# What does this PR do?

Initial implementation for `Conversations` and `ConversationItems` using
`AuthorizedSqlStore` with endpoints to:
- CREATE
- UPDATE
- GET/RETRIEVE/LIST
- DELETE

Set `level=LLAMA_STACK_API_V1`.

NOTE: This does not currently incorporate changes for Responses, that'll
be done in a subsequent PR.

Closes https://github.com/llamastack/llama-stack/issues/3235

## Test Plan
- Unit tests
- Integration tests

Also comparison of [OpenAPI spec for OpenAI
API](https://github.com/openai/openai-openapi/tree/manual_spec)
```bash
oasdiff breaking --fail-on ERR docs/static/llama-stack-spec.yaml https://raw.githubusercontent.com/openai/openai-openapi/refs/heads/manual_spec/openapi.yaml --strip-prefix-base "/v1/openai/v1" \
--match-path '(^/v1/openai/v1/conversations.*|^/conversations.*)'
```

Note I still have some uncertainty about this, I borrowed this info from
@cdoern on https://github.com/llamastack/llama-stack/pull/3514 but need
to spend more time to confirm it's working, at the moment it suggests it
does.

UPDATE on `oasdiff`, I investigated the OpenAI spec further and it looks
like currently the spec does not list Conversations, so that analysis is
useless. Noting for future reference.

---------

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

llama_stack/core/stack.py

@@ -15,6 +15,7 @@ import yaml
 
 from llama_stack.apis.agents import Agents
 from llama_stack.apis.benchmarks import Benchmarks
+from llama_stack.apis.conversations import Conversations
 from llama_stack.apis.datasetio import DatasetIO
 from llama_stack.apis.datasets import Datasets
 from llama_stack.apis.eval import Eval
@@ -34,6 +35,7 @@ from llama_stack.apis.telemetry import Telemetry
 from llama_stack.apis.tools import RAGToolRuntime, ToolGroups, ToolRuntime
 from llama_stack.apis.vector_dbs import VectorDBs
 from llama_stack.apis.vector_io import VectorIO
+from llama_stack.core.conversations.conversations import ConversationServiceConfig, ConversationServiceImpl
 from llama_stack.core.datatypes import Provider, StackRunConfig
 from llama_stack.core.distribution import get_provider_registry
 from llama_stack.core.inspect import DistributionInspectConfig, DistributionInspectImpl
@@ -73,6 +75,7 @@ class LlamaStack(
     RAGToolRuntime,
     Files,
     Prompts,
+    Conversations,
 ):
     pass
 
@@ -312,6 +315,12 @@ def add_internal_implementations(impls: dict[Api, Any], run_config: StackRunConf
     )
     impls[Api.prompts] = prompts_impl
 
+    conversations_impl = ConversationServiceImpl(
+        ConversationServiceConfig(run_config=run_config),
+        deps=impls,
+    )
+    impls[Api.conversations] = conversations_impl
+
 
 class Stack:
     def __init__(self, run_config: StackRunConfig, provider_registry: ProviderRegistry | None = None):
@@ -342,6 +351,8 @@ class Stack:
 
         if Api.prompts in impls:
             await impls[Api.prompts].initialize()
+        if Api.conversations in impls:
+            await impls[Api.conversations].initialize()
 
         await register_resources(self.run_config, impls)