Support gitlab-sshd in addition to openssh

What does this Merge Request do and why?

gitlab-shell!394 (merged) looks to implement a built-in SSH daemon for gitlab-shell. The GDK currently supports running the OpenSSH daemon but is disabled by default. A built-in SSH daemon brings a lot of benefits, with the most relevant here being the removal of needing to install OpenSSH for GDK in order to access GitLab via SSH.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• [-] gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Closes #1150 (closed)