Adds Next.js App Router example for Webhook signing

Addresses: #2100

examples/webhook-signing/nextjs/app/api/webhooks/route.ts

@@ -0,0 +1,61 @@
+import { buffer, stripe } from "@/lib/stripe";
+import { NextRequest } from "next/server";
+
+export const config = {
+    api: {
+        bodyParser: false,
+    },
+};
+
+export async function POST(req : NextRequest){
+  const sig = req.headers.get("stripe-signature")
+  const wh_sec = process.env.STRIPE_WEBHOOK_SECRET
+  let event: Stripe.Event;
+  if (!sig){
+      throw new Error("Invalid Signature")
+  }
+  try {
+      const body = Uint8Array.from(await buffer(req)) as Buffer<ArrayBufferLike>
+      event = stripe.webhooks.constructEvent(body, sig!, wh_sec)
+  }
+  catch (err : any){
+      console.log(`❌ Error message: ${err.message}`);
+      return new Response(`Webhook Error: ${err.message}`, {status : 400})
+  }
+  console.log('✅ Success:', event.id);
+
+  // Cast event data to Stripe object
+  if (event.type === 'payment_intent.succeeded') {
+    const stripeObject: Stripe.PaymentIntent = event.data
+      .object as Stripe.PaymentIntent;
+    console.log(`💰 PaymentIntent status: ${stripeObject.status}`);
+  } else if (event.type === 'charge.succeeded') {
+    const charge = event.data.object as Stripe.Charge;
+    console.log(`💵 Charge id: ${charge.id}`);
+  } else {
+    console.warn(`🤷‍♀️ Unhandled event type: ${event.type}`);
+  }
+
+  // Return a response to acknowledge receipt of the event
+  return Response.json({received: true}, {status : 200});
+}
+
+const buffer = async(req: NextRequest) => {
+    const body = req.body;
+    if (!body) {
+        throw new Error('Request body is null');
+    }
+    const reader = body.getReader();
+    const chunks = [];
+    let done, value;
+    while ({ done, value } = await reader.read(), !done) {
+        chunks.push(value);
+    }
+    const buf = new Uint8Array(chunks.reduce((acc, chunk) => acc + chunk.length, 0));
+    let offset = 0;
+    for (const chunk of chunks) {
+        buf.set(chunk, offset);
+        offset += chunk.length;
+    }
+    return buf;
+};