Add rankllm-based reranker support behind config flag

[sahel-sh]

This cl:

• Add rankllm based rerankers and integrates it to searchers behind a config flag
• Hooks up the new reranker params into oss and open ai serach clients and mcp server [The last one is untested].
• change eval files to look for run_*.json files since reranker invocations history is now also stored.
• Retrieval only script that supports retrieval of initial queries with and without reranker.

[wu-ming233]

Hello @sahel-sh, thank you for your awesome reranker addition!

Do you mind also uv add the new dependencies (e.g., rankllm), and upload the updated uv files too? Thanks!

[MXueguang]

@wu-ming233 this is ready to merge?