Custom Action support

What does this MR do?

Adds Custom Action support to api/allowed endpoint. Also brings in proper HTTP status codes returned from api/allowed.

Currently, only EE will have the ability to return a Gitlab::GitAccessResult::CustomAction.

Are there points in the code the reviewer needs to double check?

• FIXMEs removed
• Need Changelog entry?
• Need documentation change?

❗ Works with https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6456 - SSH git push to secondary -> proxy to Primary

Why was this MR needed?

To facilitate the proxying of SSH git push requests sent to the primary, from the secondary https://gitlab.com/gitlab-org/gitlab-ee/issues/6533

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• [ ] API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • [ ] Has been reviewed by a UX Designer
  • [ ] Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • [ ] Has been reviewed by a Database specialist
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [ ] Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• [ ] Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/6533