Promotions: bulk product discounting

For merchants with larger product portfolios, it would be handy to support a central place to define promotions that affect a large number of products.

These promotions can be time-limited in the same way discounts normally can, and can apply a discount to all affected products.

Promotions may be made available to storefront SDKs to let merchants build any messaging they want around it, using Moonbase as the source of truth for ongoing promotions.

They should support custom properties like other entities, to make sure merchants can attach additional data.