Add handling for paid invoices in billing services

In response to paid invoices, a new callback method has been introduced in the billing services. This feature includes changes in billing-webhook-handler.service.ts, lemon-squeezy-webhook-handler.service.ts, and other related files. The new method fetches and handles paid invoice information from Stripe and LemonSqueezy subscriptions.

packages/billing/core/src/services/billing-webhook-handler.service.ts

@@ -42,6 +42,12 @@ export abstract class BillingWebhookHandlerService {
       // one-time payments
       onPaymentFailed: (sessionId: string) => Promise<unknown>;
 
+      // this method is called when an invoice is paid. We don't have a specific use case for this
+      // but it's extremely common for credit-based systems
+      onInvoicePaid: (
+        subscription: UpsertSubscriptionParams,
+      ) => Promise<unknown>;
+
       // generic handler for any event
       onEvent?: (data: unknown) => Promise<unknown>;
     },