Implement API to get single Project Remote Mirror

Merged Timo Furrer requested to merge tuxtimo/gitlab:feature/get-project-mirror-api into master

This change implements the GET API to retrieve a single projects remote mirror.

Driver for this change is the Terraform GitLab provider which currently has to list all mirrors to just get a single specific one.

/cc @nagyv-gitlab @nmezzopera

Edited by Timo Furrer