[FIX] delivery_carrier,sale_giftcard: get the right shipping price even with gift card

Step:
- Create a product with sale price 120 USD
- Create a giftcard with balance 40 uSD
- Create a free Shipping cost of 40 USD fixed price and freeing above 100 USD
- Go to website->shop, select the product and checkout the cart(the shipping is free because price exceeds 100)
- Add the gift card to payment

Issue:
The shipping price gets from free to 40

Cause:
When computing the cost of the shipping the fact of the presence of a giftcard is not considered.

Solution:
Create a method to returns the amount of giftcard from a sale order and add to total price to check if the shipping is free

opw-3107284

closes odoo/odoo#110952

X-original-commit: 05e3f4dfc16f96cf31d9cf7afd1e25dc2f607d67
Signed-off-by: William Braeckman (wbr) <wbr@odoo.com>
Signed-off-by: Djoumatchoua Eteil Junior (etdj) <etdj@odoo.com>