feat: RAG API Server to use Async/Await

[ishaansehgal99]

Reason for Change:
Endpoints are currently sequentially blocking synchronous. After review we use async-await pattern correctly.

PR also fixes up the endpoints to be more REST-friendly - here's the three new/fixed endpoints
@app.get("/indexes", response_model=List[str]) - returns all indexes names (new endpoint)
...

@app.get("/indexes/{index_name}/documents", response_model=ListDocumentsResponse) - returns all documents in an index (new endpoint)
....

@app.get("/documents", response_model=ListDocumentsResponse) - returns all documents across all indexes

[zhuangqh]

=========================== short test summary info ============================
FAILED presets/ragengine/tests/vector_store/test_chromadb_store.py::TestChromaDBVectorStore::test_query_documents - TypeError: 'coroutine' object is not subscriptable
FAILED presets/ragengine/tests/vector_store/test_faiss_store.py::TestFaissVectorStore::test_query_documents - TypeError: 'coroutine' object is not subscriptable
================== 2 failed, 15 passed, 35 warnings in 29.66s ==================
make: *** [Makefile:109: rag-service-test] Error 1

please take a look

[Fei-Guo]

async def health_check():

As we discussed, should we remove async here?