Add GitHub Copilot OAuth provider

[Hozyne-OpenBak]

Part of #2

Implement GitHub OAuth device flow for Copilot access.

Endpoints:

• Device code: POST https://github.com/login/device/code
• Token: POST https://github.com/login/oauth/access_token

API Endpoint:

• https://api.githubcopilot.com/

Reference: https://docs.github.com/en/apps/oauth-apps/building-oauth-apps/authorizing-oauth-apps#device-flow