add schema and operations for submit cancellation feedback, along with associated enum

src/operations/payments.gql

@@ -63,3 +63,10 @@ mutation CancelSubscription {
     stripeSubscriptionId
   }
 }
+mutation SubmitCancellationFeedback(
+  $reasons: [CancellationReasonEnum!]
+  $feedback: String
+  $metadata: CancellationFeedbackMetadataInput
+) {
+  submitCancellationFeedback(reasons: $reasons, feedback: $feedback)
+}