[Feature Request]Please add rerank function

## Describe the solution you'd like
I would like to be able to use the rerank function when searching RAG.

## Why the solution needed
Because RAG search accuracy improves

## Additional context

## Implementation feasibility

- [ ] Yes, I am able to implement the feature and create a pull request.
- [x] No, I am unable to implement the feature, but I am open to discussing the solution.

The issue names no files, tests, or entry points. Start by locating the RAG search flow and documenting how search results are produced; clarify which rerank function and provider should be supported before defining implementation work. Done should include an agreed reranking behavior and tests for the resulting search accuracy or ordering.

Written by the indexing model from the issue text.