Enforce auth via SSH certificates for Git access

What does this MR do and why?

Blueprint: https://docs.gitlab.com/ee/architecture/blueprints/ssh_certificates/

• Implement an option to enforce using SSH certificates only for authentication and forbid personal SSH keys and access tokens

Related:

• Enforce authentication via SSH certificates for... (#421917 - closed)
• Extend internal endpoints to process SSH certif... (!131435 - merged)

Steps

Very similar to Extend internal endpoints to process SSH certif... (!131435 - merged)

• Check out this branch
• Run gdk start
• Enable enforce_ssh_certificates feature flag
• Go to Gitlab Shell project (cd gitlab-shell)
• Apply this diff ssh-certificates.diff
• Check out main, stop the running sshd server: gdk stop sshd
• export FF_GITLAB_SHELL_SSH_CERTIFICATES=1 to enable the Gitlab Shell feature flag
• make && bin/gitlab-sshd -config-dir=. to compile and run sshd server
• Try to git clone a project from the group: should be successful
• Go to the Group Settings > General > Permissions and group features
• Select Only SSH Certificates
• Try to git clone a project from the group: should not be successful
• Generate an SSH certificate using these ssh-keygen steps
• Add CA.pub to a group using rails c or REST get, post and delete endpoints for SSH Cer... (!130866 - merged)
• And use the certificate to git clone
• Try to git clone a project from the group: should be successful
• Go to the Group Settings > General > Permissions and group features
• Select Only SSH
• Try to git clone a project from the group: should not be successful