Extract EE::API::JobArtifacts

What does this MR do?

Extract EE::API::JobArtifacts

CE counterpart: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17556

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

I decided to leave route_setting :authentication, job_token_allowed: true in CE because, it doesn't hurt? It could be very tough to extend that.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Review
  • Has been reviewed by Backend

What are the relevant issue numbers?

Extension of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4404 and part of https://gitlab.com/gitlab-org/gitlab-ee/issues/4643