Support for ed25519-sk SSH keys

[manveru]

Is your feature request related to a problem? Please describe.

Currently the only key type for ed25519 that's supported for SSH is ssh-ed25519, which makes it impossible to use FIDO2 keys (like a Yubikey) to authenticate with the SSH server module.

Describe the solution you'd like

Please add support for the sk-ssh-ed25519@openssh.com key type.

Describe alternatives you've considered

For now I'll have to request my users to use other key types, but given the rising popularity of hardware keys this is not a very good option.

Additional context

The key type has been implemented in OpenSSH for over 4 years, current implementation is at https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/ssh-ed25519-sk.c?rev=1.15&content-type=text/x-cvsweb-markup&only_with_tag=MAIN