MakeCredential should allow u2f mode

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