Proposal for adding conversation history

[glennfeys]

As mentioned in #302 There is no easy way to include message history eventhough this could quite easily be added in a couple of ways. Since it's a logical next step I am unsure if @jamescalam already has on opinion on how to add this? Some ways that feel obvious to me is by adding a preprocessor step that could do one of both things:

1. Rewrite the last message by providing n LLM with the history and telling it to rewrite it if necesairy for a standalone question. Downsides of this is that it has a drastic impact on speed eventhough a small model could already be used to do this rewriting.
2. Embedding both question and question+history, then treat these two as seperate async router calls and based on the similarity scores of the question and question+history encoding you can check if question has high confidence you can pick this routing if not you can use the one with history. Or you can even use this in the aggregation per group, similar to HybridRouter.

Hope we can have a discussion what would fit here best since I think many people are hoping for this feature and is necessary to have a decent router in a continuous interactive process like a chatbot.