fix/parse-auth-data-extension-data

[MasterKale]

Issue #124 highlighted how the library incorrectly parses a response's authenticatorData. For example, data containing extension data would return a malformed credential public key containing the extension data.

This diff enhances authenticator data parsing to always correctly return the credential public key, and also return the CBOR-encoded bytes structure representing any included extension data.

NOTE: Parsing extension data into something intelligible will come later when I get around to addressing Issue #104. For now if you want the extension data in authenticatorData (instead of what is returned from getclientextensionresults() in the browser) you can do something like this:

import cbor2
from webauthn.helpers import parse_authenticator_data
from webauthn.helpers.structs import RegistrationCredential  # Should work for AuthenticationCredential too

credential = RegistrationCredential.parse_raw(credential_json)
parsed_auth_data = parse_authenticator_data(credential.response.authenticator_data)
parsed_extension_data: dict = cbor2.loads(parsed_auth_data.extensions)