[IMP] sale_product_configurator: 'Product Variant' field optionally visible

-Product configurator applies to all a db. If need it for just a few products.
 If want to scan or type sku of variants of other products to quickly populate the order.
 it's time-consuming to go through product configurator.

-In this commit, To overcome of this issue, 'Product Variant' field optionally
 visible when product_configurator is installed.

task- 2082126
closes- odoo#41979

closes odoo/odoo#41979

Signed-off-by: Damien Bouvy (dbo) <dbo@odoo.com>