authenticateUser
Starts authentication flow. If [registeredAuthenticatorId] is null, starts authentication by default authenticator. Usually it is Pin authenticator.
var userId = await Onegini.instance.userClient
.authenticateUser(context, registeredAuthenticatorId)
.catchError((error) {
print("Authentication failed: " + error.message);
});
if (userId != null) {
print("Authentication success!");
}
