Support SSH certificates

Description

SSH supports certificates. Gitlab should allow using a CA key as a SSH key.

~"feature proposal"

Proposal

Theoretically, I think the only change that is needed is to relax the validation to ignore a "cert-authority" option at the start of a key. SSH should already know how to do all the heavy lifting wrt certificate authentication, and the gitlab-shell service is invoked with the correct key id.

Links / references

• man ssh-keygen(1), section CERTIFICATES
• man sshd(8), section AUTHORIZED_KEYS FILE FORMAT, specifically the part about the "cert-authority" option.