💡 idea: Allow using OpenAI's assistants API

[se4n01]

Description

The assistants api provides backend for vector maps and also for functions. Instead of integrating these yourself, which is a lot of work, you can just hook the assistants API as an option for type of bot. This way, users can create their rich assistants using proprietary data and system prompts and mattermost can use them without changing the plugin much. This is a strategy for dealing with #241 in a clean and easy way.

Relevant code as POC: https://github.com/ryo-ma/gpt-assistants-api-ui

Best wishes,

Sean