chore(firestore-stripe-invoices): added validation for payment due days configuration (#361)

Author: dackers86

firestore-stripe-invoices/extension.yaml

@@ -159,6 +159,9 @@ params:
       What is the default number of days the customer has before their payment is due?
       The invoice automatically closes after this number of days.
       You can override this default value for each invoice.
+    validationRegex: "^[0-9]+$"
+    validationErrorMessage: >
+      Please enter a valid number
     default: 7
     required: true