MC-19254: Implement the schema changes

- chagning cart prices discount to total_item_discount

Author: Prabhu Ram

app/code/Magento/QuoteGraphQl/Model/Resolver/CartItemPrices.php

@@ -70,7 +70,7 @@ public function resolve(Field $field, $context, ResolveInfo $info, array $value
                 'currency' => $currencyCode,
                 'value' => $cartItem->getRowTotalInclTax(),
             ],
-            'discount' => [
+            'total_item_discount' => [
                 'currency' => $currencyCode,
                 'value' => $cartItem->getDiscountAmount(),
             ],

app/code/Magento/QuoteGraphQl/etc/schema.graphqls

@@ -333,7 +333,7 @@ type CartItemPrices {
     row_total: Money!
     row_total_including_tax: Money!
     discounts: [Discount] @doc(description:"An array of discounts to be applied to the cart item")
-    discount: Money @doc(description:"The total of all discounts applied to the item")
+    total_item_discount: Money @doc(description:"The total of all discounts applied to the item")
 }
 
 type SelectedCustomizableOption {

dev/tests/api-functional/testsuite/Magento/GraphQl/Quote/CartPromotionsTest.php

@@ -88,7 +88,7 @@ private function assertLineItemDiscountPrices($response, $productsInCart, $qty,
                     'prices' => [
                         'row_total' => ['value' => $productsInCart[$itemIndex]->getSpecialPrice()*$qty],
                         'row_total_including_tax' => ['value' => $productsInCart[$itemIndex]->getSpecialPrice()*$qty],
-                        'discount' => ['value' => $productsInCart[$itemIndex]->getSpecialPrice()*$qty*0.5],
+                        'total_item_discount' => ['value' => $productsInCart[$itemIndex]->getSpecialPrice()*$qty*0.5],
                         'discounts' => [
                             0 =>[
                                 'amount' =>
@@ -169,7 +169,7 @@ public function testCartPromotionsMultipleCartRules()
 
             //removing the elements from the response so that the rest of the response values can be compared
             unset($productsInResponse[$itemIndex][0]['prices']['discounts']);
-            unset($productsInResponse[$itemIndex][0]['prices']['discount']);
+            unset($productsInResponse[$itemIndex][0]['prices']['total_item_discount']);
             $this->assertResponseFields(
                 $productsInResponse[$itemIndex][0],
                 [
@@ -253,7 +253,7 @@ public function testCartPromotionsSingleCartRulesWithTaxes()
                         // row_total including tax is the price + price * tax rate
                         'row_total_including_tax' => ['value' => $rowTotalIncludingTax],
                         // discount from cart rule after tax is applied : 50% of row_total_including_tax
-                        'discount' => ['value' => round($rowTotalIncludingTax/2, 2)],
+                        'total_item_discount' => ['value' => round($rowTotalIncludingTax/2, 2)],
                         'discounts' => [
                             0 =>[
                                 'amount' =>
@@ -285,7 +285,7 @@ private function getCartItemPricesQuery(string $cartId): string
         row_total_including_tax{
           value
         }
-        discount{value}
+        total_item_discount{value}
         discounts{
           amount{value}
           label