Feature Request: Support ed25519-sk and ecdsa-sk key types

[ellenhp]

I'd like to use my FIDO2 resident keys to authenticate with ssh_ui [1] applications, so this would be a cool thing to have. Protocol and key format seem to be defined here [2]. I might look into this next time I get the urge to work on my BBS system, but until then I thought I'd put up an issue to track it and pool brainpower in case there's someone else out there who wants this feature.

[1] https://github.com/ellenhp/ssh_ui
[2] https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.u2f