MakeCredential should allow u2f mode

[kazzmir]

A yubikey can be forced in u2f mode by invoking

fido_dev_force_u2f(token);

MakeCredential should invoke this before calling fido_dev_make_cred, perhaps if MakeCredentialOpts had a ForceU2F=true field or some such. In u2f mode, a pin is not required.

fido_dev_force_u2f: https://github.com/Yubico/libfido2/blob/b99038c391792546185235032776987147959e27/src/fido.h#L49