Expose the Koding application settings in the API

What does this MR do?

Exposes koding_enabled and koding_url in the settings api endpoint

Why was this MR needed?

When saving the GitLab application secrets in Koding, and authorising your admin user to have access to the UI, we want to let Koding enable the integration, and populate the url in GitLab for the user.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/22705