Provide a way to easily use aiohttp-devtools for AsyncApp apps #165

Originally, this is a question in the community workspace:
https://community.slack.com/archives/CHY642221/p1606598878089000

Currently, the AsyncApp#start() does not work along with aiohttop-devtools, which is the project's official autoload tool for aiohttp. The reason is mentioned in the project's readme:

note: since adev adev runserver will import whole file so it doesn't work with web.run_app(app), or use if name == 'main': web.run_app(app)

https://github.com/aio-libs/aiohttp-devtools/blob/v0.13.1/README.rst#runserver

I'm still thinking about the design but AsyncApp can provide a handy way for this use case.

Category (place an x in each of the [ ])

• slack_bolt.App and/or its core components
• slack_bolt.async_app.AsyncApp and/or its core components
• Adapters in slack_bolt.adapter
• Others

Requirements

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