Discount list should be per cart line item id, not (just) per variant ID

[mariusa]

The same variant ID can be in cart on multiple lines, with different attributes. But Shopify functions output requires to specify a variant ID instead of cart line item id:

"targets": [
        {
          "productVariant": {
            "id": "gid://shopify/ProductVariant/1"
          }
        }
      ],

To keep compatibility, another target type could be added (eg cartLineItem), besides productVariant.

Also requested by other partners on forums, ignored by Shopify: https://community.shopify.com/c/shopify-functions/feature-request-add-ability-to-discount-a-specific-cart-line/td-p/1801254