Allow a custom SocketModeReceiver to be used with Socket Mode

[zimeg]

Summary

This PR allows a custom SocketModeReceiver to be used with Socket Mode when initializing an app.

Reviewers

These changes can be verified with the following snippets:

Socket Mode receiver

This should now work fine! The custom route at http://localhost:3000/health and the message response should both respond with something expected.

/** Socket Mode receiver */
const receiver = new SocketModeReceiver({
    appToken: process.env.SLACK_APP_TOKEN,
    customRoutes: [
    {
      path: '/health',
      method: ['GET'],
      handler: (req, res) => {
        res.writeHead(200);
        res.end(`Things are going just fine at ${req.headers.host}!`);
      },
    },
  ]
});

/** App initialization */
const app = new App({
    token: process.env.SLACK_BOT_TOKEN,
    socketMode: true,
    appToken: process.env.SLACK_APP_TOKEN,
    logLevel: LogLevel.DEBUG,
    receiver
});

app.message('hello', async ({ say }) => {
  await say('hi!');
});

(async () => {
    await app.start(process.env.PORT || 3000);
    console.log("⚡️ Bolt app is running!");
})();

Express receiver

This will still error with an AppInitializationError trying to run the app.

/** Express receiver */
const receiver = new ExpressReceiver({
    signingSecret: process.env.SLACK_SIGNING_SECRET,
});

receiver.router.use((req, res, next) => {
    console.log(`Request time: ${Date.now()}`);
    next();
})

receiver.router.get("/health", (req, res) => {
    res.writeHead(200);
    res.end(`Things are going just fine at ${req.headers.host}!`);
});

/** App initialization */
const app = new App({
    token: process.env.SLACK_BOT_TOKEN,
    socketMode: true,
    appToken: process.env.SLACK_APP_TOKEN,
    logLevel: LogLevel.DEBUG,
    receiver
});

app.message('hello', async ({ say }) => {
  await say('hi!');
});

(async () => {
    await app.start(process.env.PORT || 3000);
    console.log("⚡️ Bolt app is running!");
})();

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #1972 (1b59b70) into main (2b259a9) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1972   +/-   ##
=======================================
  Coverage   82.21%   82.21%           
=======================================
  Files          18       18           
  Lines        1524     1524           
  Branches      438      438           
=======================================
  Hits         1253     1253           
  Misses        175      175           
  Partials       96       96           

Files	Coverage Δ
src/App.ts	83.41% <100.00%> (ø)

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[seratch]

LGTM 👍

[filmaj]

LGTM! Was this in response to a particular issue?

[zimeg]

@seratch @filmaj Thank y'all for the reviews! This was found while exploring for #1971 though doesn't directly address the issue.