Modular RAG - Part 2

[ThomasVitale]

This is part 2 of the Modular RAG solution started in #1603

The scope I'm focusing on for the next milestone is the following. I've started already the several tasks, but I'll submit PRs step-by-step.

• Support similarity scores in Document API Support similarity scores in Document API #1794
• Introduce ScoringModel API
• Support Cohere for chat, embedding, and scoring
• Add implementations of DocumentJoiner and DocumentRanker based on similarity scores
• Consolidate Documents metadata and use them in QueryAugmenter implementation
• Review conditional and branching components in Modular RAG
• Add Modular RAG documentation

[ThomasVitale]

Closing this for now. The only pending task is the introduction of a new rerank/score Model implementation, which will be addressed in a separate task.