Adding SIMPLE product enum

SIMPLE subscription product enum was missing, thus the yaml file has been updated to add it into the product.cs file.

Author: rlayhbrian

xero-app-store.yaml

@@ -469,11 +469,13 @@ components:
             * FIXED: Customers are charged a fixed amount for each billing period
             * PER_SEAT: Customers are charged based on the number of units they purchase
             * METERED: Customers are charged per use of this product
+            * SIMPLE: Customers are charged on a fixed amount for each billing period with an optional add-on feature
           type: string
           enum:
           - FIXED
           - PER_SEAT
           - METERED
+          - SIMPLE
         usageUnit:
           description: The unit of the usage product. e.g. "user", "minutes", "SMS", etc
           type: string