[FIX] mrp{,_subcontracting}: Add parent_product in bom overview

Before this, when working on a bom with variants, when checking the
parent from the component / child_bom, it would always take the first
variant since it used the bom's product_template.

This could lead to incorrect data when working with subcontracting, as
when another variant from the first would be selected, when fetching
quantities no parent info would be found as it would seek info from the
first variant of the bom instead of the correct one.

By doing this, we can now have the right variant and fetch the right
informations.

Part of task-2985735

Part-of: odoo/odoo#104893