Add OpenAI API compatible assistant

[nenb]

PoC - do not merge.

Details

1. Download https://huggingface.co/Mozilla/Mistral-7B-Instruct-v0.2-llamafile/blob/main/mistral-7b-instruct-v0.2.Q5_K_M.llamafile (or host your own model locally)
2. export BASE_URL=http://localhost:8080 (or wherever your model is available locally)
3. Add assistants = [ "ragna.assistants.OpenAIApiCompatible", ] to ragna.toml