Add support for FIDO2 pubkey auth for SSH server

[adiroiban]

This is to implement the userauth server-side of https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.u2f

The security key (sk-) implementation is for now only OpenSSH specific.

The GitHub.com server (SSH-2.0-babeld-70f1bac9 but maybe OpenSSH in the backend) also supports it.

For the scope of this ticket, my plan is to have support only for:

• [email protected]
• [email protected]

This means:

• Load public blob
• validate signature

Certificates are out of scope. WebAuthN is out of scope.

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In the past, getting yubikey to work with SSH was a pain via PGP or SmartCard

With this, OpenSSH has direct access to the fido2 keys.