feat: multi-query retrieval

[mhordynski]

Feature description

Ragbits' DocumentSearch should provide a way to perform multi-query retrieval.

Proposed approach:

• Create new QueryRephraser which uses LLM to generate multiple queries from one question.
• Create new Reranker which gathers results from different queries and reranks them based on that how often given Element repeates in query results.

Motivation

Multi-querying is popular method of improving retrieval performance.

Additional context

No response

[ludwiktrammer]

For the reranker we should probably use something similar to Reciprocal Rank Fusion (as Qdrant do), so that we boost the one that appear multiple times, without discarding the information about positions within the individual result list.