Add ChatbackendInterface

Problem/Motivation

Currently the AI Assistant API is coupled with the AI Chatbot, but we should be able to decouple this by adding an ChatbackendInterface that normalizes the request/response methods and make it possible for any backend to hook in and use the Chatbot.

That way anyone wanting for instance to do naive RAG can do that, without the whole overhead of Assistants or anyone wanting to interact directly with one specific agent.

Proposed resolution

Add an interface (perhaps plugin for discoverability).
Figure out the actual input and outputs needed for request/response.

Remaining tasks

User interface changes

API changes

Data model changes