Implement the shipment entity type

Items represent what’s inside the shipment. My current thinking is to have them serialized ala adjustments, each ShipmentItem is a value object that has a purchased entity type, purchased entity id, quantity. We also talked about storing an order item ID but at this point I’m no longer convinced it’s necessary. Might just need a use case for it though.

The reason why I want to serialize items is because they’re going to be recalculated during the refresh. If our default behavior is “single shipment per order”, then each change to the order (at cart/checkout) must be reflected in the shipment.

The amount is a price field, taken from the selected rate. Adjustments store handling fees and/or taxes.