T9251: accept OpenSSH sk-* key types in vyos_user
VyOS has accepted sk-ecdsa-sha2-nistp256@openssh.com and
sk-ssh-ed25519@openssh.com since T4750, but the module's choices
list was never updated, so a playbook using a security key fails
argspec validation while the equivalent CLI commands succeed.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>