Improve PromotionRule.gifts field by returning ProductVariant types instead of IDs.

[zedzior]

After implementing this, field PromotionRule.gifts returns a list of ProductVariant IDs:

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

Would be good to return ProductVariant types instead, like:

type PromotionRule implements Node @doc(category: "Discounts"){
  ...
  gifts: [ProductVariant!]
  ...
}

The problem is ProductVariant type exists in channel context and PromotionRule can be assocciated with multiple channels.

One of the solution is to return some kind of narrowed ProductVariant type, which excludes all the fields, which derive from ProductVariantChannelListing.