Add Support for App Manifest Endpoints

[misscoded]

Summary

This pull request introduces support for the App Manifest endpoints, including:

apps.manifest.create
apps.manifest.delete
apps.manifest.export
apps.manifest.update
apps.manifest.validate
tooling.tokens.rotate

WebAPICallResult should be switched out when the API responses become available and the generated types are updated.

Requirements (place an x in each [ ])

• 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.

[seratch]

The changes look good to me. If you have more time, can you add a test script under prod-server-integration-tests for easily running end-to-end tests with the endpoints? As the endpoints are not yet available in production, we can set the dev environment base URL from an env variable as below:

const { WebClient } = require('@slack/web-api');
const options = {};
if (process.env.SLACK_SDK_TEST_DEV_API_URL !== undefined) {
  options.slackApiUrl = process.env.SLACK_SDK_TEST_DEV_API_URL;
}
const client = new WebClient(process.env.SLACK_SDK_TEST_TOOLING_TOKEN, options);
// perform API calls below ...

[misscoded]

@seratch For the test script, I kept the bot token as the "main" token, and used the tooling token in an override manner, as I assume this is how most folks would use it in practice. Let me know if you think otherwise

[seratch]

As long as both ways (1: passing in constructor 2: passing as argument) work, it's fine.