Add Github Copilot integration so I don't have to pay for yet more subscriptions

### Summary of the new feature / enhancement

I should be able to opt to use my github copilot subscription for AI Shell and have it use my existing token alotments for github copilot. You should be able to just select the right modesl from it.

### Proposed technical implementation details (optional)

On startup add copilot to the options not just Azure and ChatGPT.