Add support for "sub apps"

When creating a large slack application it would be nice to be able to reuse components in a more pythonic way than what is possible right now.
A workaround is described in issue #236, but it requires passing the app object around and manually wiring up things by registering and calling decorators as functions directly.

I suggest adding a feature much like include_router in FastAPI or add_typer in Typer.

This feature will let us create sub-apps in separate files or even separate packages, like plugins that can be published on pypi.org separately.

Here is a small example illustrating what I mean:

main.py

Here is our main app, where we add a plugin from a separate package

import os
from slack_bolt import App

from my_plugin import my_plugin


# Initializes your app with your bot token and signing secret
app = App(
    token=os.environ.get("SLACK_BOT_TOKEN"),
    signing_secret=os.environ.get("SLACK_SIGNING_SECRET")
)
app.add_app(my_plugin)

my_plugin.py

A simple plugin that just creates a slack_bolt app, but without any config

from slack_bolt import App


my_plugin = App()

@my_plugin.message("hello")
def message_hello(message, say):
    say("Hello world")

Category

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

Start by reviewing the slack_bolt.App and slack_bolt.async_app.AsyncApp composition APIs, then read the workaround described in issue #236. Compare the requested add_app behavior with FastAPI's include_router and Typer's add_typer. The work is done when independently defined sub-apps or plugins can be attached to both app variants without manually passing the parent app or invoking decorators directly.