Add a method for loading the order from the order management API

Problem/Motivation

Currently, KlarnaManager::getOrder() and KlarnaManager::loadOrder() both load the order from the Klarna Checkout endpoint.

After the customer has completed the purchase, the order needs to be loaded from the order management API.

Updating loadOrder() to do this now isn't an option for backwards compatibility reason. I'm suggesting we add a loadCompletedOrder() method to the KlarnaManager.