feat(#651): sort by discount

[novoj]

We should build on issue #549 and introduce a new sort option that would allow to specify a list of price lists that should be considered as discounted. The query engine should then calculate the discounted price according to the inner record price handling rules and calculate the discount compared to the selling price:

{
  queryProduct(
    filterBy: {
      entityPrimaryKeyInSet: [
        103885
      ],
      priceInCurrency: EUR,
      priceInPriceLists: [
        "employee-basic-price",
        "basic"
      ]
    }
    orderBy: {
      priceDiscount: {
        order: ASC
        inPriceLists: [
          "reference",
          "basic"
        ]
      }
  ) {
    recordPage {
      data {
        primaryKey
        attributes {
          code
        }
        priceForSale {
          priceWithoutTax
          priceWithTax
          taxRate
        }
        referencePrice: price(priceList: ["reference", "basic") {
          priceWithoutTax
          priceWithTax
          taxRate
        }
      }
    }
  }
}

And in evitaQL:

query(
    collection("Product"),
    filterBy(
        priceInPriceLists("basic"),
        priceInCurrency("EUR"),
        priceBetween(100, 103)
    ),
    orderBy(
        priceDiscount(ASC, "reference", "basic")
    )
    require(
      entityFetch(
        attributeContent("code"),
        priceContentRespectingFilter()
      )
    )
)

If the discount result is negative (i.e. the discounted price is greater than the selling price), it's considered to be zero. Sorting in ascending order means that the products with no discount are returned first, and the products with the largest discount are returned last. Descending order returns the product with the largest discount first.

❗ Important change

The sellable property on the PriceContract interface has been renamed to indexed because it better describes its purpose. It's also more aligned with property naming in the attribute schema - which also uses the `indexed' keyword for the same purpose.