[WIP] version 6 main features

[whitead]

• [x] Generalizes context to be multipart messages
• [x] Allows multimodal contexts
• [x] Enables caching control on messages
• [x] Adjusts flow to allow context to be either packed into system prompt or some separate part of query, so that it can be cached
• [ ] Update typing/update litellm types
• [ ] Create image context that encodes image to a URL
• [ ] Add a PDF reader that uses images instead of text
• [x] Deprecate detailed evidence

[whitead]

Moved to #704