Specify multi-invoices shipping/discounts/computing

[LouiseBonnard]

Closed because issues need to be specified, we need to clarify the expected behavior for multi invoices regarding shipping fees management and discounts. The main problem is that we only have ONE Cart for multiple OrderInvoice so we have to manage weird computations to split the total among several invoices

One possible (and clean) solution would be to have one dedicated cart per invoice although it requires quite some changes in the database and model But we need to decide:

1. does each invoice have its dedicated shipping cost
2. can each invoice have a different Carrier
3. this means each invoice must have its Delivery slip
4. ... probably other issues ^^

Originally posted by @jolelievre in https://github.com/PrestaShop/PrestaShop/pull/20776#issuecomment-712271177