Refactored classes according to new convention

packages/billing/gateway/src/server/services/billing-gateway/billing-gateway.service.ts

@@ -14,6 +14,12 @@ import {
 
 import { BillingGatewayFactoryService } from './billing-gateway-factory.service';
 
+export function createBillingGatewayService(
+  provider: z.infer<typeof BillingProviderSchema>,
+) {
+  return new BillingGatewayService(provider);
+}
+
 /**
  * @description The billing gateway service to interact with the billing provider of choice (e.g. Stripe)
  * @class BillingGatewayService
@@ -23,7 +29,7 @@ import { BillingGatewayFactoryService } from './billing-gateway-factory.service'
  * const provider = 'stripe';
  * const billingGatewayService = new BillingGatewayService(provider);
  */
-export class BillingGatewayService {
+class BillingGatewayService {
   constructor(
     private readonly provider: z.infer<typeof BillingProviderSchema>,
   ) {}