Added 'discountTypeCombination' field to Cart and Order

We've introduced a new field, discountTypeCombination, that allows you to check how discounts apply and interact on a Cart and Order. Additionally, the new field also indicates the discount type chosen when the best deal logic applies to the Cart or Order.

Changes:

• [API] Added the discountTypeCombination field to Cart and Order.
• [API] Added DiscountTypeCombination, BestDeal, and Stacking types to Carts API.
• [GraphQL API] Added the discountTypeCombination field to the Cart and Order types.
• [GraphQL API] Added the following types to the GraphQL schema: DiscountTypeCombination, Stacking, BestDeal, ChosenDiscountType.

extend type Cart {
  discountTypeCombination: DiscountTypeCombination
}

extend type Order {
  discountTypeCombination: DiscountTypeCombination
}

"Chosen discount type for the cart as part of best deal"
enum ChosenDiscountType {
  CartDiscount
  ProductDiscount
}

interface DiscountTypeCombination {
  type: String!
}

type Stacking implements DiscountTypeCombination {
  type: String!
}

type BestDeal implements DiscountTypeCombination {
  type: String!
  chosenDiscountType: ChosenDiscountType
}