API for configuring repository push mirroring via SSH
### Problem to solve
We support HTTP and SSH push mirroring via the UI but not the API only.
### Further details
Adding SSH support is more difficult because of the backwards and forwards interaction required to configure it.
### Proposal
Implement API support for SSH push mirroring
### Links / references
https://docs.gitlab.com/ee/user/project/repository/repository_mirroring.html#pushing-to-a-remote-repository-core
issue