Add ability to alter the the information sent to the gateway before redirection

Problem/Motivation

I want to send a custom sermepa order ID. Right now it is hardcoded to
$gateway->setOrder(substr($date->format('ymdHis') . 'Id' . $order->id(), -12, 12))

Proposed resolution

Add a hook_commerce_sermepa_gateway_alter to be able to do manipulations before encoding the parameters for sending.

Remaining tasks

Agree on the approach and code the solution.

User interface changes

None

API changes

New hook

Data model changes

None