Adjust API for gift promotion rule

[IKarbowiak]

Part of #15075

API changes

Mutations

• Update the promotion inputs with gifts field
• Update the following mutations: promotionCreate, PromotionRuleCreate, promotionRuleUpdate:
  • Require the gifts field when the GIFT reward type is chosen
  • rewardValueType, rewardValue, rewardPredicate should be empty - raise an ValidationError otherwise
  • Validate gifts field - the provided IDs must be the ProductVariant ids
• Update CheckoutLinesUpdate - don't allow to update quantity of lines marked as gift line
• Update OrderLineUpdate - don't allow to update quantity of lines marked as gift line
• Update checkoutLineDelete and CheckoutLinesDelete - do not allow deleting gift lines

input PromotionRuleInput @doc(category: "Discounts") {
  ...
  gifts: [ID!]
}

input PromotionRuleCreateInput @doc(category: "Discounts") {
  ...
  gifts: [ID!]
}

input PromotionRuleUpdateInput @doc(category: "Discounts") {
  ...
   gifts: [ID!]
}

Types

• Extend PromotionRule with giftIds
• Extend PromotionRule withgiftsLimit - will define the maximum number of gifts to choose from the gifts list. For now, we will always return 1, and it won't be configurable for now
• Extend RewardTypeEnum with GIFT
• Extend OrderLine and CheckoutLine with isGift flag
• Consider adding filter for getting lines that are gifts

type PromotionRule implements Node @doc(category: "Discounts"){
  ...
  giftIds: [ID!]
  giftsLimit: Int
}

enum RewardTypeEnum @doc(category: "Discounts") {
  ...
  GIFT
}

type OrderLine implements Node & ObjectWithMetadata @doc(category: "Orders") {
  ...
  isGift: Boolean!
}

type CheckoutLine implements Node & ObjectWithMetadata @doc(category: "Checkout") {
  ...
  isGift: Boolean!
}