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

[jeromepochat]

This encapsulate fields making possible to extend GitHubAppCredentials to retrieve properties from an alternative storage instead of field values.

Pseudo code:

class AlternativeGitHubAppCredentials extends GitHubAppCredentials {

    @Override
    public getAppID() {
        return retrieveFromExternalSystem("appId");
    }

    @Override
    public getPrivateKey() {
        return retrieveFromExternalSystem("privateKey");
    }
}

See JENKINS-73388 for further information.

[car-roll]

LGTM

[jglick]

#803