[5.X] Allow the default variant to be selected per site/store

[benfeather]

We manage a Craft Commerce website with dedicated stores for different regions, including New Zealand, Australia, the United Kingdom, and the United States.

Our products are available across all stores, but some items have variants specific to each region. These variants may include local power adapters, user guides, and other region-specific features.

Currently, Craft Commerce only allows the selection of one default variant, which is shared across all stores.

For example, the 70ml with NZ/AU charger is the default variant for our NZ store:

However, we cant choose the 70ml with UK charger as the default variant for our UK store: