Allow ECommerce::processFeedback() to be overridden in subclass

Problem/Motivation

\Drupal\commerce_ingenico\Plugin\Commerce\PaymentGateway\ECommerce::processFeedback() implements most of the business logic connecting Ingenico and the Commerce payment. However, this method cannot be overridden in a subclass to implement additional logic since it is declared as a private method.

Proposed resolution

Convert \Drupal\commerce_ingenico\Plugin\Commerce\PaymentGateway\ECommerce::processFeedback() to a protected method, so it can be overridden by a subclass.

Remaining tasks

1. Write a patch
2. Review
3. Commit

User interface changes

None.

API changes

None.

Data model changes

None.