Implement new billing-gateway and update related services

Created a new package named billing-gateway which implements interfaces for different billing providers and provides a centralized service for payments. This will potentially help to maintain cleaner code by reducing direct dependencies on specific payment providers in the core application code. Additionally, made adjustments in existing services, like Stripe, to comply with this change. The relevant interfaces and types have been exported and imported accordingly.

packages/billing-gateway/src/components/embedded-checkout.tsx

@@ -0,0 +1,39 @@
+import { lazy } from 'react';
+
+import { Database } from '@kit/supabase/database';
+
+type BillingProvider = Database['public']['Enums']['billing_provider'];
+
+export function EmbeddedCheckout(
+  props: React.PropsWithChildren<{
+    checkoutToken: string;
+    provider: BillingProvider;
+  }>,
+) {
+  const CheckoutComponent = loadCheckoutComponent(props.provider);
+
+  return <CheckoutComponent checkoutToken={props.checkoutToken} />;
+}
+
+function loadCheckoutComponent(provider: BillingProvider) {
+  switch (provider) {
+    case 'stripe': {
+      return lazy(() => {
+        return import('@kit/stripe/components').then((c) => ({
+          default: c.StripeCheckout,
+        }));
+      });
+    }
+
+    case 'lemon-squeezy': {
+      throw new Error('Lemon Squeezy is not yet supported');
+    }
+
+    case 'paddle': {
+      throw new Error('Paddle is not yet supported');
+    }
+
+    default:
+      throw new Error(`Unsupported provider: ${provider as string}`);
+  }
+}