Fix slackapi#1358 Expose common utilities for building HTTP module ba…

…sed receivers

examples/custom-receiver/.gitignore

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+.env
+dist/
+node_modules/
+package-lock.json

examples/custom-receiver/README.md

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+# Bolt for JavaScript Koa Receiver Example App
+
+This is a quick example app to demonstrate how to implement a custom receiver to integrate `App` with 3rd party web framework, which allows developers to directly access Node.js http package interface.
+
+## Install Dependencies
+
+To link to latest source code, you can run the following script:
+
+```
+./link.sh
+```
+
+## Setup Environment Variables
+
+This app requires you setup a few environment variables. You can find these values in your [app configuration](https://api.slack.com/apps).
+
+```bash
+export SLACK_CLIENT_ID=YOUR_SLACK_CLIENT_ID
+export SLACK_CLIENT_SECRET=YOUR_SLACK_CLIENT_SECRET
+export SLACK_SIGNING_SECRET=YOUR_SLACK_SIGNING_SECRET
+```
+
+## Run the App
+
+Start the app with the following command:
+
+```
+npm start
+```
+
+### Running with OAuth
+
+Only implement OAuth if you plan to distribute your application across multiple workspaces. Uncomment out the OAuth specific comments in the code. If you are on dev instance, you will have to uncomment out those options as well.
+
+Start `ngrok` so we can access the app on an external network and create a redirect URL for OAuth.
+
+```
+ngrok http 3000
+```
+
+This output should include a forwarding address for `http` and `https` (we'll use the `https` one). It should look something like the following:
+
+```
+Forwarding   https://3cb89939.ngrok.io -> http://localhost:3000
+```
+
+Then navigate to **OAuth & Permissions** in your app configuration and click **Add a Redirect URL**. The redirect URL should be set to your `ngrok` forwarding address with the `slack/oauth_redirect` path appended. ex:
+
+```
+https://3cb89939.ngrok.io/slack/oauth_redirect
+```
+
+Start the OAuth flow from https://{your own subdomain}.ngrok.io/slack/install

examples/custom-receiver/link.sh

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+#!/bin/bash
+
+current_dir=`dirname $0`
+cd ${current_dir}
+npm unlink @slack/bolt \
+  && npm i \
+  && cd ../.. \
+  && npm link \
+  && cd - \
+  && npm i \
+  && npm link @slack/bolt

examples/custom-receiver/package.json

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+{
+  "name": "bolt-oauth-example",
+  "version": "1.0.0",
+  "description": "Example app using OAuth",
+  "main": "app.js",
+  "scripts": {
+    "lint": "eslint --fix --ext .ts src",
+    "build": "npm run lint && tsc -p .",
+    "build:watch": "npm run lint && tsc -w -p .",
+    "start": "npm run build && node dist/app.js"
+  },
+  "license": "MIT",
+  "dependencies": {
+    "@koa/router": "^10.1.1",
+    "@slack/logger": "^3.0.0",
+    "@slack/oauth": "^2.5.0-rc.1",
+    "dotenv": "^8.2.0",
+    "koa": "^2.13.4"
+  },
+  "devDependencies": {
+    "@types/koa__router": "^8.0.11",
+    "@types/node": "^14.14.35",
+    "ts-node": "^9.1.1",
+    "typescript": "^4.2.3"
+  }
+}