KVBench - Add support for different random distributions (random, uniform, zipfian) when selecting chunks for RAG usecase

[wvaske]

When selecting chunks for RAG, the current implementation does a np.random.choice to get retrieved_indices in RAGDocumentManager.retrieve_chunks().

There should be an option to define a different random distribution to change the hit rate histogram of chunks

Priority:
Zipfian or Power

Others:
uniform