Copy LFS objects when a repository is push mirrored over SSH

This is a follow-up issue to #16525 (closed) and !40137 (merged) , in which we add LFS sync support to push mirroring over HTTPS only.

It would be good to support push mirroring over SSH as well, but to be able to do this, we need to work out how to acquire the necessary metadata and credentials.

The git lfs push command would make an SSH connection and run a custom command (we support this via gitlab-shell) to do so - is it wise to do the same from within Ruby + Sidekiq?