Artifact repository specific subdomain

While we are enhancing the existing artifact API to support the Maven and Debian protocols, it is not easy to manually use. This is because the API endpoints are likely to be fairly deep endpoints off the main GitLab hostname.

To make this easier to use, to hand out, to remember, etc. we should allow it to run on its own subdomain with direct access to the API endpoint.

For example it should be possible to add packages.gitlab.com, or similar, and get direct access to the desired API's.

This would work in conjunction with our upcoming instance level support, : https://gitlab.com/gitlab-org/gitlab-ee/issues/7769