changePin
Start the change pin flow to change a users pin.
Future<void> changePin() 
await Onegini.instance.userClient
    .changePin()
    .catchError((error) {
    if (error is PlatformException) {
        print("Pin change failed");
    }
});
During the change pin flow the following events will be fired after which changePin will resolve:
- OpenPinAuthenticationEvent; answer using the 
OneginiPinAuthenticationCallback().acceptAuthenticationRequest("pin") - ClosePinAuthenticationEvent
 - OpenPinRegistrationEvent; answer using 
OneginiPinRegistrationCallback().acceptAuthenticationRequest("newPin") - ClosePinRegistrationEvent