Fix #1038 by making the AwsLambdaReceiver example TS compatible

examples/deploy-aws-lambda/app.js

@@ -57,6 +57,6 @@ app.message('goodbye', async ({ message, say }) => {
 
 // Handle the Lambda function event
 module.exports.handler = async (event, context, callback) => {
-  const handler = await app.start();
+  const handler = await awsLambdaReceiver.start();
   return handler(event, context, callback);
 }

src/App.ts

@@ -433,6 +433,7 @@ export default class App {
   public start(
     ...args: Parameters<HTTPReceiver['start'] | SocketModeReceiver['start']>
   ): ReturnType<HTTPReceiver['start']> {
+    // TODO: HTTPReceiver['start'] should be the actual receiver's return type
     return this.receiver.start(...args) as ReturnType<HTTPReceiver['start']>;
   }
 

src/receivers/AwsLambdaReceiver.spec.ts

@@ -8,6 +8,7 @@ import AwsLambdaReceiver from './AwsLambdaReceiver';
 import crypto from 'crypto';
 import rewiremock from 'rewiremock';
 import { WebClientOptions } from '@slack/web-api';
+import { AwsHandler } from './AwsLambdaReceiver';
 
 describe('AwsLambdaReceiver', function () {
   beforeEach(function () {});
@@ -48,7 +49,7 @@ describe('AwsLambdaReceiver', function () {
       const awsReceiver = new AwsLambdaReceiver({
         signingSecret: 'my-secret',
       });
-      const handler = await awsReceiver.start();
+      const handler: AwsHandler = await awsReceiver.start();
       assert.isNotNull(handler);
     });