Introduced fixed price cart discounts as beta feature

Introduced a new Cart Discount type for fixed prices on line items. With this you can now specify to which price you want to discount the line items, so far you could define the amount or percentage to be deducted from the line item price only.
With this new discount type, you can model cart promotions like these:

1. Final Sale - all articles of the 'Sales' category in the cart for 15€ each
2. Buy a pair of sunglasses for 10€ with coupon code SUNGLASSES (not limited to one pair, all pairs will become 10€)
3. Phone accessories are only $10 with the purchase of any new phone (not limited to one accessory, all accessories will become $10)
4. Get any 2 shirts for $50 each (not limited to two shirts, all shirts will become $50 if there are at least 2 shirts in the cart).

• [API] Added CartDiscountValue of type fixed that can be set on the CartDiscountValueDraft.
• [GraphQL API] Added the fixed field to the CartDiscountValueInput type.

Introduced the following changes to the GraphQL schema (in SDL format):

extend type CartDiscountValueInput {
  fixed: FixedPriceDiscountValueInput
}