hidgd: get all the way through an authenticator transaction

Still using dummy values.  However, the design is to get

cargo run --example main

of gecko-dev/dom/webauthn/u2f-hid-rs all the way through an
authenticator transaction, even if the returned values are currently
bogus (the main.rs example doesn't check the values).

Next step is to add cryptography

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>