feat: allow specifying specific tool within toolgroup (#1239)

Summary:

E.g. `builtin::rag::knowledge_search`

Test Plan:
```
LLAMA_STACK_CONFIG=fireworks pytest -s -v tests/client-sdk/agents/ --safety-shield meta-llama/Llama-Guard-3-8B
```

llama_stack/distribution/ui/page/playground/rag.py

@@ -132,7 +132,7 @@ def rag_chat_page():
         },
         toolgroups=[
             dict(
-                name="builtin::rag",
+                name="builtin::rag/knowledge_search",
                 args={
                     "vector_db_ids": [vector_db_id for vector_db_id in selected_vector_dbs],
                 },