PROMO-1050 Add as_total field to shipping discount action

Author: bc-luke

reference/promotions.v3.yml

@@ -831,6 +831,12 @@ components:
           properties:
             discount:
               $ref: '#/components/schemas/Discount'
+            as_total:
+              type: boolean
+              description: |-
+                Set this value to true to distribute the discount as a total among shipping destinations. By default, the discount applies to each destination.
+                Example: If set to false, the discount is $10 and you have 2 shipping destinations eligible for this discount, the shipping cost for both destinations will be discounted by $10, with a total of $20 off the order.
+                If set to true, $10 will be distributed among the 2 shipping destinations, weighted by their respective price. In a case where there are 2 destinations sharing the same cost, each will be discounted by $5.
             method_ids:
               description: ''
               oneOf: