add support for generic open ai assistant api

[prabirshrestha]

We should look into bringing open ai assistant api that works with any LLMs.

Reference:

• https://github.com/metaskills/experts and https://news.ycombinator.com/item?id=40395107
• https://github.com/supercorp-ai/supercompat
• https://platform.openai.com/docs/assistants/overview