Support passwordless authentication

[Eldemarkki]

Adding support for passkeys/WebAuthn/FIDO2 (not sure what their differences are) would be a dream come true.

[lajp]

I remember reading some discussion of this over at vaultwarden. Can't find it right now but I got the impression that this is a non-trivial task and I'm unsure if all necessary libraries for this even exist in Rust.