This adds server-side support for the newly introduced OpenSSH keytypes sk-ecdsa-sha2-nistp256@openssh.com and sk-ed25519@openssh.com (including their corresponding certificates), which are backed by U2F/FIDO2 tokens.
The format for the new key types and their signatures can be found at https://raw.githubusercontent.com/openssh/openssh-portable/master/PROTOCOL.u2f