[DFC Orders] Updates to backorder on any order amendment

[mkllnk]

Currently, we amend the backorder when a customer checks out an order or at the end of the order cycle. But there are many more ways a backorder can be influenced, including:

• A customer amends an order by adjusting quantities.
• A customer cancels an order.
• An admin creates an order.
• An admin amends an order by adding items, removing items or changing quantities.
• An admin adjusts stock levels.
• A subscription places an order.

There may be more ways to affect a backorder and it's difficult to catch all cases.

So far, the backordering logic is tailored to the checkout and order cycle close but we need a more general approach to cover all situations.

• We need to load all line items of the order cycle to find new lines to add to the backorder.
• We need to load all variants in the order cycle to find out-of-stock items to order.
• We still need to iterate through all backordered items to check if they are still needed. A product could have been ordered, then cancelled and removed from the order cycle.

Once we have a general method aggregating all backorder information we can then use it various places.

[mkllnk]

I think that this work depends on:

• [DFC Orders] Store links to open backorders #12952

When variants were removed from an order cycle and removed from orders then we don't have any way of finding an open backorder associated to the removed variant unless we catch the exact event of removal.