Added variant.key to predicate field identifiers

It is now possible to use variant.key as a Discount Predicate field identifier. This allows you to use the same Discount Predicate for your production environment as you have used on your staging environment. Before it was not always possible since you had to rely on the automatically generated product.id and variant.id instead that differ between Projects.

• [API] Added variant.key field to Product Field Identifiers.
• [API] Added variant.key field to LineItem Field Identifiers.