Missing API endpoint for upstream (pull) mirror URL for projects mirroring

Proposal

It is currently not possible to pull the upstream (pull) mirror URL for projects configured for mirroring. This prevents users and administrators from querying their projects for what upstream mirroring dependencies exist.

It is possible to find the downstream (push) mirrors for a project: https://docs.gitlab.com/ee/api/remote_mirrors.html#list-a-projects-remote-mirrors.