fix(backend): Export missing Billing types

Author: mauricioabreu

.changeset/warm-steaks-count.md

@@ -0,0 +1,5 @@
+---
+'@clerk/backend': minor
+---
+
+Export missing Billing types

packages/backend/src/index.ts

@@ -100,9 +100,13 @@ export type {
   PaginatedResponseJSON,
   TestingTokenJSON,
   WebhooksSvixJSON,
+  BillingPayerJSON,
   BillingPlanJSON,
   BillingSubscriptionJSON,
   BillingSubscriptionItemJSON,
+  BillingPaymentAttemptWebhookEventJSON,
+  BillingSubscriptionItemWebhookEventJSON,
+  BillingSubscriptionWebhookEventJSON,
 } from './api/resources/JSON';
 
 /**