issuer (str): The `iss` claim. If unspecified the current issuer
                claim will be used.
            subject (str): The `sub` claim. If unspecified the current subject
                claim will be used.
            additional_claims (Mapping[str, str]): Any additional claims for
                the JWT payload. This will be merged with the current
                additional claims.

        Returns:
            google.auth.jwt.OnDemandCredentials: A new credentials instance.
        NŠ