Add "Propagation Method" option to commerce variants

[benfeather]

When setting up a product on a craft installation with multiple sites, you can choose how the product entries propagate, like so:

It would be great if we could set the propagation method for product variants (independently of the product).

For example, I have a craft installation with sites in New Zealand, Australia, and the USA.

When I create a product available in all three countries, the content/URL should remain consistent across all the sites as the product's features don't change.

The product has two variants: one is designed for New Zealand and Australia and includes the appropriate wall adapter, while the other is specifically for the USA. Each variant has a separate SKU.

Without a variant propagation option, I would have to set the product propagation method to "Save product to all sites" and disable the variant depending on the country. This method works, but deleting the US variant on the NZ/AU sites and vice versa while keeping the same product entry would be nice.

Here's a more complicated example we have set up: