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

## 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.

https://github.com/user-attachments/assets/28a848f2-77ee-4dec-9ed3-9dd06c65f21e

## 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.

Start by reviewing Apache Answer's plugin system, configuration handling, syncer integration, and REST API architecture. The proposal's done state would include an accepted Copilot chat plugin plus dependent plugin configuration support and chat APIs, but the issue does not identify files, tests, or a settled implementation scope.

Written by the indexing model from the issue text.