Add BM25 Index

[svilupp]

• Added a keyword-based search similarity to RAGTools to serve both for baseline evaluation and for advanced performance (by having a hybrid index with both embeddings and BM25). See ?RT.KeywordsIndexer and ?RT.BM25Similarity for more information.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 88.71473% with 36 lines in your changes are missing coverage. Please review.

Project coverage is 92.69%. Comparing base (af8b238) to head (62bdb3b).

Files	Patch %	Lines
src/Experimental/RAGTools/retrieval.jl	77.10%	19 Missing :warning:
src/Experimental/RAGTools/types.jl	90.74%	10 Missing :warning:
src/Experimental/RAGTools/preparation.jl	88.23%	4 Missing :warning:
ext/RAGToolsExperimentalExt.jl	97.91%	1 Missing :warning:
ext/SnowballPromptingToolsExt.jl	90.90%	1 Missing :warning:
src/Experimental/RAGTools/utils.jl	97.14%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #157 +/- ## ========================================== - Coverage 92.98% 92.69% -0.29% ========================================== Files 42 43 +1 Lines 3308 3574 +266 ========================================== + Hits 3076 3313 +237 - Misses 232 261 +29 ```

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