[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>

addons/sale_product_configurator/views/sale_views.xml

@@ -35,7 +35,8 @@
                 <field name="is_configurable_product" invisible="1" />
             </xpath>
             <xpath expr="//tree/field[@name='product_id']" position="attributes">
-                <attribute name="invisible">1</attribute>
+                <attribute name="optional">hide</attribute>
+                <attribute name="string">Product Variant</attribute>
             </xpath>
         </field>
     </record>