Add copilot chat plugin to leverage AI to answer user queries based on questions and answers

[hgaol]

Is your feature request related to a problem? Please describe

It's actually a proposal. I think it's very suitable to add AI ability to Apache Answer cause it's an QnA platform. I think we can implement it as a plugin but I'd like to put issue here for more audience. I've already done some POC, feel free to check below demo. Welcome any comments and discussions.

Answer-Copilot-Chat.mp4

Describe the solution you'd like

As showed in above demo.

There're also some changes needed in answer repo, e.g.

1. supporting some plugin configs depend on others, and only show when it's a specific value.
2. it could be a new plugin type, cause it needs pass the syncer and add some REST API for chats.

If it's accepted, I can also implement these as well as this plugin itself.