User defined key attribute on Shipping Zones

Shipping Zones now have a user defined key attribute. The key attribute is used when creating or updating a ShippingMethod to identify a Zone.

• [API] Added the key attribute on the Zone and ZoneDraft representations.
• [API] You can now get, update, and delete a Shipping Zone by its key.
• [API] When creating a ShippingMethod, the Shipping Zone can be referenced by its ID or its key.
• [API] addShippingRate, removeShippingRate, addZone, and removeZone, can now reference a zone by its ID or its Key.