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 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.