Add support for the new pure SSH-based LFS protocol

ℹ️ Support the pure SSH-based LFS protocol for git... (&11872 - closed) is tracking the work here now.

Now that a pure SSH-based protocol has been merged in Git LFS (https://github.com/git-lfs/git-lfs/pull/4446), it would be great if GitLab could support this.

This will help in environments in which SSH is the only supported/permitted protocol, and in environments where having traffic go over HTTPS is cumbersome, for example environments where HTTPS traffic is protected by SSL client certificate authentication.