Support LEANN in llamaindex

[yichuan-w]

What problem does this solve?

To deliver to more user

Proposed solution

Add a PR to llamaindex intergration

Example usage

[yichuan-w]

We have been using ChromaDB and PGVector, we want to give LEANN a try while keeping the same incremental LLamaIndex RAG ingestion pipeline and hybrid search interface (https://developers.llamaindex.ai/python/examples/retrievers/bm25_retriever/#hybrid-retriever-with-bm25--chroma)

[ArshadJafri]

Hey @yichuan-w can i work on this..

[yichuan-w]

Sure, go ahead, #224 has partially done sth, but I think that PR is not that high quality, you can make a new PR yourself, thanks for the contribution!!