Support GitHub App authentication for Git integration

This is a fairly important need for feature parity with Azure DevOps. Being forced to use user PAT tokens is not best practice as you're more likely to run into API rate limits, it's tied to a user account so if they leave it breaks, syncs have that same user as if they made those changes and yes you can use a machine account instead but that requires a license in GitHub. Also, with automation, a user account PAT can sometimes produce false positives with the GitHub API. I've seen this happen several times when using a PAT over a GitHub app with Terraform.