Support the AppID extension for FIDO migration

[jcjones]

It'd be great to be able to exercise the AppID extension in the tests here, too.

[cpiper]

I would like to do this, but the issue is enrolling keys with an appid, which isn't supported in WebAuthn. I'll think about ways to go about this, perhaps implementing a U2F enrollment flow that will enroll with the appid.