add first class discount_subtotal var (#9112)

packages/core/types/src/order/common.ts

@@ -1149,6 +1149,11 @@ export interface OrderDTO {
    */
   tax_total: BigNumberValue
 
+  /**
+   * The discount subtotal of the order.
+   */
+  discount_subtotal: BigNumberValue
+
   /**
    * The discount total of the order.
    */