Change return type of CredentialsContainer.create for WebAuthn

[asmsuechan]

Hi developers,

The return value for CredentialsContainer.create is one of FederatedCredential, PasswordCredential, and PublicKeyCredential according to https://developer.mozilla.org/en-US/docs/Web/API/CredentialsContainer/create#return_value

The return types are subclass of Credential. So I changed using PublicKeyCredential from Credential. Note that FederatedCredential and PasswordCredential haven't been implemented yet, therefore, I didn't use them. I'll also try to create them after I learn the manners of this repo.

Thank you for reading my PR.