Cross-merchant bundles

Many Moonbase merchants already perform some cross-promotions between brands, where products are given away or sold as part of promotional campaigns. However, this is all manually done, using less than ideal tools. We could consider building out the features to let bundles have products from multiple Moonbase merchants inside, where we can split the payment accordingly between different merchants. This would allow for a much smoother customer journey too, although some features would not necessarily be available, like partial bundles (cannot check ownership of products from a different merchant).