Add support for optional authenticator_attachment in PublicKeyCredential

[8ma10s]

Why

https://w3c.github.io/webauthn/#iface-pkcredential

Level 3 draft of WebAuthn adds an optional parameter authenticatorAttachment on PublicKeyCredential (and AuthenticatorAttestationResponse and AuthenticatorAssertionResponse which inherits it, of course).

This field allows RP developers to detect whether the authentication was done using platform authenticator that always exists on that particular device, or cross-platform authenticator that only exists on that device temporarily.

In the latter case of using cross-platform authenticator, RP can prompt the user to register a platform authenticator so that the user won't lose the ability to sign in on that device.

Note: If, as the result of a registration or authentication ceremony, authenticatorAttachment's value is "cross-platform" and concurrently isUserVerifyingPlatformAuthenticatorAvailable returns true, then the user employed a roaming authenticator for this ceremony while there is an available platform authenticator. Thus the Relying Party has the opportunity to prompt the user to register the available platform authenticator, which may enable more streamlined user experience flows.

Since some vendors (I confirmed with Mac chrome) are already passing this optional parameter authenticatorAttachment, I want this gem to be able to support reading values from that field.

What

Allow initializing PublicKeyCredential class with an optional argument authenticator_attachment

Misc

I obtained the authenticator response returned from the client on sign-in (using dev-console), and ran WebAuthn::Credential.from_get on that response. As you can see, I can now obtain the value of authenticator_attachment