Support GitHub Enterprise Server (GHES)

### Feature summary

Add login support for GitHub Enterprise Server (GHES) in Copilot App.

### What problem are you trying to solve?

I’m trying to login to our GHES instance with the Copilot App but it’s currently not supported. Would be great to populate the repo fields with repos from GHES, have Copilot create PR:s etc.

### Proposed solution

Add initial support for signing in with GHES and keep adding more features over time.

### Workflow impact

All Copilot users working primarily against repos on GHES.

### Installation context

_No response_

### Additional context

Previous issues were marked as ”bug” and later closed with a clarification that Copilot App currently only supports ghe.com. This is a FR.