Keys in Cart and Product Discount predicates

We added the capability to use user-defined keys of categories, products, and customer groups in predicates for the specification of product and cart discounts. With this it is no longer required to use their platform-generated IDs in the predicates.

• [API] Added the capability to use Category key in LineItemPredicate Fields.
• [API] Added the capability to use Product key in LineItemPredicate Fields.
• [API] Added the capability to use Product key in ProductDiscount Predicate Identifiers.
• [API] Added the capability to use CustomerGroup key in LineItemPredicate Fields.
• [API] Added the capability to use CustomerGroup key in Cart-related Predicate Fields.
• [API] Added the capability to use CustomerGroup key in ProductDiscount Predicate Identifiers.