Add ability to apply custom logic for order item cloning and availability checking.

Problem/Motivation

We are using the stock module, and we want to be able to restrict adding of some products to the cart. We can just override the controller and copy-paste everything, but I don't like it.

Another thing is that we are using commerce_vado module. It adds a lot of complexity. It extensively uses data field in order_items. Hence we need to somehow clean up the order item cloning. It could be done in ORDER_CLONED, but as for me, it should be triggered only once and not in the loop.

Proposed resolution

Add two more events to be able to react on order item cloning and order item availability check.