DiscountCode can now be grouped

Discount codes now can be grouped together by assigning one or more groups to an individual discount code. Furthermore, the new Return StateRole can now be used to mark the state of those line items that belong to the return process of an order.

• [API] Added field groups to DiscountCode and DiscountCodeDraft.
• [API] Added Change Groups update action to manage the groups on a discount code.
• [API] Added ReturnStateRole for LineItems.