POC: Stop using langchain for Azure Search and OpenAI calls

[adamdougal]

• This is POC code, it's not unit tested and if we proceed it may be structured differently
• Retrains langchain as an overall orchestrator
• Motivation for doing this is to give us full control over the search index and how we query it, allowing us to add an image vector field

Required by https://github.com/Azure-Samples/chat-with-your-data-solution-accelerator/issues/748

[ross-p-smith]

I'm very happy with the direction of this - it actually simplifies things and removes a lot of langchain! Which will ultimately help @cecheta !! :-D I think we need Komal's in first though.