FIDO2 : Support username-less authentication workflow

[nynymike]

One of the nice features of FIDO2 is that it could be used for a flow without username. Could you an identifier in the key itself? Perhaps we need a seperate script for usernameless flow?

[mtgluu]

I like the idea of a separate script to drive the password-less acr. It would allow more flexibility in what to tie-in for the username value.