Support ssh-certificates

Is your feature request related to a problem? Please describe. Some people or orgs manage their ssh keys using ssh-certificates

Describe the solution you'd like Allow the admin to specify ssh-keys that may sign ssh-certificates for users. Gitea for example allows using the emails/username of user to be used as principals to check requests. I suggest simply using the usernames as principals. Later it would also be awesome to allow users to specify their own cert-authority only for themselves and to use the principals to restrict keys only to some repos using the principals.