Support repository and deployment URL of GHES

[int128]

Issue

• Deployment URL cannot be parsed for GitHub Enterprise #865

[Oded-B]

Any chance we can get this merged?

I just wrote an identical PR because I missed this one :)

I had to add /api/v3 to the newOAuth2Client BaseURL to get it to work with my instace:
https://github.com/Oded-B/argocd-commenter/pull/1/files

The /api/v3 path is automatically added to the "regular" REST calls so the GITHUB_ENTERPRISE_URL doesn't need to include it, maybe it would make more sense to add this logic to https://github.com/int128/oauth2-github-app