Implement integration with Payments API

Problem/Motivation

The module already provides the checkout process with Payments using commercetools API, that should be enough for regular cases. But some projects probably want to implement a custom payment process.

Proposed resolution

To implement this, we can try to create integration with some Drupal Payments API modules, to not develop separate modules for each payment service. Or look at the Commerce Payments modules and try to add support for them.