Perhaps https://github.com/hub4j/github-api.

The main motivation is that right now we don't handle errors correctly, i.e. we often assume that the calls succeed.

• Add Library -- refactor: replace custom HTTP client with GitHub API library when fetching installation token #1889
• fetching GitHub app auth token -- refactor: replace custom HTTP client with GitHub API library when fetching installation token #1889
• Implement it to all routes