Create helper method to addDonationItem

I was able to gracefully get the donation pane working on my own checkout flow by simply adding a donation item to all new orders inside of an EventSubscriber that reacts on OrderEvents::ORDER_CREATE. The cleanest way to do this was to just copy the bit logic from NewOrder->get() that handles adding the order item to the order. Attaching a patch that just breaks out that bit of logic into its own NewOrder->addDonationItem() method, allowing it to be implemented elsewhere via the service.