[FIX] sale_product_configurator: Track custom values when adding an item to cart from the website

Steps to reproduce the issue:
- Go to the website shop
- Select a product which allows custom features (eg. Customizable desk)
- Select custom and enter a value
- Add to cart

Issue:
Neither the modal preview nor the cart of the added item doesn't show the custom value, also it
doesn't appear in the created sale order

Cause:
The optional sale_product_configurator module hasn't implemented it

opw-3052987

closes odoo/odoo#106784

Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>