Support for making Jenkins a "GitHub App"

To my understanding currently the github-branch-source plugins always requires GitHub user credentials / tokens to authenticate.

I'd suggest to add authenticating Jenkins to GitHub as a GitHub App too.

Why is this better than the current way:

• GitHub Apps can be granted very fine grained permissions
• GitHub Apps can be added either to a whole org, or just to selected repos
• The app uses a key pair to then get temporary credentials, so leaked creds to user are only valid for a short period of time
• Higher API limits! (probably the most important one for bigger orgs)

This is specifically NOT about authenticating users against GitHub, but for Authenticating Jenkins itself against GitHub

References: https://developer.github.com/apps/differences-between-apps/