[JENKINS-73388] Allow alternative implementation for GitHub App credentials

[jeromepochat]

The current Connector supports GitHub App credentials with class inheritance constraint on credentials implementation.

This PR introduces an interface to support GitHub App credentials alternative implementations that cannot extends GitHubAppCredentials.

See JENKINS-73388 for further information.

Submitter checklist

• Link to JIRA ticket in description, if appropriate.
• Change is code complete and matches issue description
• Automated tests have been added to exercise the changes
• Reviewer's manual test instructions provided in PR description. See Reviewer's first task below.

Reviewer checklist

• Run the changes and verify that the change matches the issue description
• Reviewed the code
• Verified that the appropriate tests have been written or valid explanation given

Documentation changes

• Link to jenkins.io PR, or an explanation for why no doc changes are needed

Users/aliases to notify

[jeromepochat]

Closed as superseded by #796