feat: OpenAIVectorIOMixin for vector_stores common logic (#2427)

Extracts common OpenAI vector-store code into its own mixin so that all
providers can share the same core logic.
This also makes it easy for Llama Stack to support both vector-stores
and Llama Stack APIs in the interim so that both share the same
underlying vector-dbs.

Each provider contains storage specific logic to `create / edit / delete
/ list` vector dbs while the plumbing logic is standardized in the
common code.

Ensured that this works well with both faiss and sqllite-vec. 

### Test Plan 
```
llama stack run starter
pytest -sv --stack-config http://localhost:8321 tests/integration/vector_io/test_openai_vector_stores.py --embedding-model all-MiniLM-L6-v2
```

llama_stack/apis/vector_io/vector_io.py

@@ -165,7 +165,7 @@ class VectorIO(Protocol):
     @webmethod(route="/openai/v1/vector_stores", method="POST")
     async def openai_create_vector_store(
         self,
-        name: str | None = None,
+        name: str,
         file_ids: list[str] | None = None,
         expires_after: dict[str, Any] | None = None,
         chunking_strategy: dict[str, Any] | None = None,