roadmap: Jan implements `/assistants` that can have Tools

## Goal 

- Assistants can use Tools
    - MCP called from `chat/completions`
    - We do not use Runs

## Tasklist

### Frontend
- [ ] Deprecate assistant extension retrieval tool
- [ ] Use file_search and defile tool use

### Backend
- [ ] Support file_search tool & vectorDB
- [ ] Support `tools` in chat/completions (v1)
- [ ] Support /runs to process chat_completions itself (v2)