Support for GitHub Apps with username / password

[klopfdreh]

Is your feature request related to a problem? Please describe.
We need the functionality that Spring Cloud Config Server supports GitHub Apps, so that we are able to configure the access to our Repository more fine-grained.

Describe the solution you'd like
Within the GitCredentialsProviderFactory we can extend the username / password mechanism to retrieve an GitHub access token.

Describe alternatives you've considered
N/A

Additional context
N/A

I am going to provide a PR for this soon

[klopfdreh]

PR: #3189

[spencergibb]

Closing in favor of #3188

[klopfdreh]

@spencergibb - I am looping 😉