Initial endpoint logic to create a customer return
Reimburse customer returns created through the add_customer_return_handler
Update customer return handler reimbursement creation

* build reimbursements from the customer return so only non-reimbursed accepted return items get reimbursed
* don't expect a reimbursement for return items for something other than the rma expected per https://github.com/spree/spree/commit/1bcd2792c18d64a83ee4925720f80227a36de30d#diff-e00e5798905985d9f2ce3fb833b17ce4R6
Don't attempt to reimburse customer returns for orders with non-inventory refunds

* These require special handling/calculation, so we don't want to auto-reimburse them
Make shipment serializer build its items from inventory units

Currently shipments build their items from the line items, not the
inventory units. For normal shipments that does not pose a problem,
however, for exchange shipments (https://github.com/spree/spree/pull/5073),
where a new inventory unit is generated for the original line item
(but a new variant), the original item's sku would end up being included in
the shipment payload, which is incorrect.

The information sent in the shipment record ends up being mostly the
same, with the difference being that until inventory_units have a
quantity on them (coming soon https://github.com/spree/spree/issues/5091)
items will be flattened.
Add returns related serializers so they can be pushed.

Recommended:
Push CustomerReturn and Reimbursement separately, the former to do
accurate inventory reconciliation as needed and look for manual
intervention cases. The latter for financial reconciliation against an
order.
Fix inventory unit serializer to use line item price
Add resolution_path to customer returns

For customer returns that are not fully reimbursed, it is important to
provide a link to customer service agents notified through wombat. The
url is non-standard (as customer_returns are not intended to be scoped
to orders for the long term), so it is actually less brittle to build it
here.
Add item_inventory_units method in add customer return handler