Support job-dsl native syntax for folder-scoped github app credentials

[justusbunsi]

Description

These changes allows users to easily define folder-scoped github app credentials via job-dsl plugin. Without it, users would have to use the all-mighty "configure" callback to add those settings by xml modification. Sample job-dsl of what now works:

organizationFolder('sample') {
    organizations {
        github {
            repoOwner("jenkinsci")
            credentialsId("<credentials-id>")
        }
    }
    properties {
        folderCredentialsProperty {
            domainCredentials {
                domainCredentials {
                    domain {
                        name('folder.sample')
                        description('Folder-scoped credentials')
                    }
                    credentials {
                        // 👇 THIS HERE IS THE IMPORTANT PART
                        gitHubApp {
                            id('folder-scoped-github-app-credentials')
                            description('GitHub App for accessing repositories')
                            appID("...")
                            privateKey(Secret.fromString("""..."""))
                            repositoryAccessStrategy {
                                specificRepositories {
                                    owner('organisation-name')
                                    repositories(['repo-1', 'repo-2'])
                                }
                            }
                            defaultPermissionsStrategy('CONTENTS_READ')
                            scope('GLOBAL')
                        }
                        // ☝️ THIS HERE IS THE IMPORTANT PART
                    }
                }
            }
        }
    }
}

I am not sure if you want a test for this, and if it is even possible. Let me know if it is. I am happy to add one.

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

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