Resident credential support

[P4sca1]

Recent versions of Google Chrome and Microsoft Edge support Resident keys.
This allows you to login without a username.
To use the feature we need to change an attribute in authenticatorSelection when registering the credential, which is not possible at the moment. Would be great if this library would be more flexible with the options we can give to generateRegistrationChallenge.

Also would be great if somebody could provide an example on what needs to be changed on the frontend code to support this. Afaik we no longer need the challengeRequest from the server, but I have no idea how to get the credentials.