Support Github Copilot Enterprise provider.

[carseven]

Currently opencode only support Copilot auth using the standard domain github.com, but when using github enterprise on premise account this does not work. We need to provide a custom domain.

I am interested to contribute if needed, and here we have the VSCODE official implementation of the github auth that supports enterprise accounts https://github.com/microsoft/vscode/tree/main/extensions/github-authentication so the support should be easy.