Move OffsitePaymentGatewayInterface::onNotify to its own interface

We are developing a payment gateway in Bolivia that uses cash payments in stores (https://www.drupal.org/project/commerce_pagos_net). The plugin is implementing both the OnsitePaymentInterface (because during checkout we call a web service to send the code that the user will have to present at the physical store to pay) and also implements the OffistePaymentInterface, because the payments provider sends a JSON back to our site once the user pays in any store. It works right now because in the PaymentProcess class, the `if` checks before if we are implementing the OnsitePaymentInterface, but if the if order changed then it would no longer work. I think it would be better to separate the onNotify method of the OffsitePaymentInterface to another interface so that any payment gateway can implement the onNotify method and use the OffsitePaymentController.