Shopify is taking a big step forward in its much-watched advertising business.The e-commerce firm is formally rolling out a product that allows its merchants to recommend products on other merchants’ storefronts. The new feature, Product Network, expands Shopify’s three-year-old ad format called Shop Campaigns that merchants use to acquire customers. Up until now, merchants have only been able to run Shop Campaigns within Shopify’s consumer-facing Shop app and website. Now, those campaigns can a...