text: Title: Add a ranker component that uses an LLM to rerank documents
  Body: **Describe the solution you'd like**
  I’d like to add a new ranker component that leverages a LLM to rerank retrieved documents based on their relevance to the query. This would better assess the quality of the top-ranked documents, helping ensure that only relevant results are given to the LLM to answer the question. 
  
  Additionally, having an ability for the LLM to choose how many documents to keep would also be nice. A sort of dynamic top-k if you will. 
  
  **Additional context**
  We have started to employ this for some clients especially in situations where we need to provide extensive references. Basically for a given answer we need to provide all relevant documents that support the answer text. Having one reference in these situations is not enough. As a result in these situations we are willing to pay the extra cost to use an LLM to rerank and only keep the most relevant documents. 
  