Custom product types: support for variations.

[AnnaMag]

WooCommerce 7.9.0 introduced a change to the "Use with variations" checkbox on the Edit Product page which prevents merchants from being able to create e.g. a Variable Subscription product.

Here's the relevant changes in WooCommerce that caused this:
WooCommerce PR #38802 Changed the Used for variations checkbox code and adds a new enable_if_variable className
WooCommerce PR #39106 Causes the Used for variations checkbox to be disabled for variable subscription products

When trying to create a variable subscription product, the "use with variations" checkbox on the attributes page is getting disabled which prevents creating variations for subscriptions:

This was fixed for Subscriptions, we should test on other custom products.

Related report: #39338.