Selecting a GitHub Copilot account different from the GitHub account for a given project

### Feature summary

Use a GitHub Copilot account different from the GitHub account to work on projects

### What problem are you trying to solve?

I have two GitHub accounts - one for personal projects, and one for work. I frequently use my personal GitHub account to run experiments before they are "promoted" to an official repository under my employer's GitHub organization. This leads to a situation where I frequently use my *work* GitHub Copilot account to work on projects that need to use my *personal* GitHub account for things like git, gh cli, pull requests, etc. In other words - tokens paid by my employer that I'm free to use to run the wildest experiments I can think of, but the GitHub account itself doesn't match the GitHub Copilot account.

### Proposed solution

Allow selecting the GitHub Copilot and GitHub accounts separately on a per-project basis. This is not an issue in VSCode because the GitHub and GitHub Copilot extensions allow selecting accounts separately, this is how I've been doing it so far, but the Copilot app doesn't offer this kind of granular controls yet.

### Workflow impact

It's annoying, and a drag to productive usage of Copilot app. I have to use the work account for Copilot but then it will keep failing to open PRs or make calls to GitHub, so I manually instruct it to use gh cli with the right account explicitly, and work around it that way. I want this to just work without the repeated access denied errors due to the incorrect user being used to call GitHub.

### Installation context

_No response_

### Additional context

_No response_