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
```

docs/_static/llama-stack-spec.html

@@ -12583,6 +12583,9 @@
                     }
                 },
                 "additionalProperties": false,
+                "required": [
+                    "name"
+                ],
                 "title": "OpenaiCreateVectorStoreRequest"
             },
             "VectorStoreObject": {

docs/_static/llama-stack-spec.yaml

@@ -8791,6 +8791,8 @@ components:
           description: >-
             The provider-specific vector database ID.
       additionalProperties: false
+      required:
+        - name
       title: OpenaiCreateVectorStoreRequest
     VectorStoreObject:
       type: object