Add support for one-time PayPal donations

service/src/main/graphql/braintree/TokenizePayPalOneTimePayment.graphql

@@ -0,0 +1,8 @@
+# https://graphql.braintreepayments.com/reference/#Mutation--tokenizePayPalOneTimePayment
+mutation TokenizePayPalOneTimePayment($input: TokenizePayPalOneTimePaymentInput!) {
+  tokenizePayPalOneTimePayment(input: $input) {
+    paymentMethod {
+      id
+    }
+  }
+}