[FIX] l10n_it_edi: Discount computation from invoice import

This fixes the how general discounts and product discounts are represented in the Italian EDI.

Previously, discounts were erroneously calculated and represented on several invoice lines not linked to the product.
Now, cascading discounts are summaries as 1 percentage discount for products and 1 amount for the general discount.

closes odoo/odoo#85796

Task: 2764978
Signed-off-by: Josse Colpaert <jco@odoo.com>