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>

tests/unit/conversations/test_api_models.py

@@ -0,0 +1,60 @@
+# 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.
+
+
+from llama_stack.apis.conversations.conversations import (
+    Conversation,
+    ConversationCreateRequest,
+    ConversationItem,
+    ConversationItemList,
+)
+
+
+def test_conversation_create_request_defaults():
+    request = ConversationCreateRequest()
+    assert request.items == []
+    assert request.metadata == {}
+
+
+def test_conversation_model_defaults():
+    conversation = Conversation(
+        id="conv_123456789",
+        created_at=1234567890,
+        metadata=None,
+        object="conversation",
+    )
+    assert conversation.id == "conv_123456789"
+    assert conversation.object == "conversation"
+    assert conversation.metadata is None
+
+
+def test_openai_client_compatibility():
+    from openai.types.conversations.message import Message
+    from pydantic import TypeAdapter
+
+    openai_message = Message(
+        id="msg_123",
+        content=[{"type": "input_text", "text": "Hello"}],
+        role="user",
+        status="in_progress",
+        type="message",
+        object="message",
+    )
+
+    adapter = TypeAdapter(ConversationItem)
+    validated_item = adapter.validate_python(openai_message.model_dump())
+
+    assert validated_item.id == "msg_123"
+    assert validated_item.type == "message"
+
+
+def test_conversation_item_list():
+    item_list = ConversationItemList(data=[])
+    assert item_list.object == "list"
+    assert item_list.data == []
+    assert item_list.first_id is None
+    assert item_list.last_id is None
+    assert item_list.has_more is False