feat: add [async] optional dependencies for easier async setup

[yukiyan]

Summary

Add [async] optional dependencies to pyproject.toml so users can install async requirements with pip install "slack-bolt[async]" instead of manually installing aiohttp.

Changes:

• Add optional-dependencies.async to pyproject.toml (references existing requirements/async.txt)
• Update install instructions in README.md, docs/, and slack_bolt/async_app.py docstring

Category

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

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.
• I've run ./scripts/install_all_and_run_tests.sh after making the changes.

[yukiyan]

@WilliamBergamin@lukegalbraithrussell@zimeg Could you review this PR when you get a chance? Thanks!

[WilliamBergamin]

Hi @yukiyan thanks for opening this 💯

Currently we recommend running pip install slack_bolt aiohttp to use the AsyncApp, making pip install "slack_bolt[async]" reference the import in requirements/async.txt may not be the desired behavior as requirements/async.txt is intended for testing.

We might want to think more about this strategy before implementing a slack_bolt[async] 🤔 since it means that this project would support specific versions of aiohttp and bumping the minimum version may result in breaking changes here

[yukiyan]

Sorry for the late reply, @WilliamBergamin!

That's a fair point — I hadn't considered the maintenance cost of supporting specific aiohttp versions as optional dependencies. Bumping the minimum version could indeed introduce breaking changes, which adds ongoing burden to the maintainers.

I was inspired by python-slack-sdk's optional.txt and thought a similar approach could work here, but I understand that the context is different — requirements/async.txt in bolt-python is intended for testing, not for end-user dependency management.

I'll go ahead and close this PR. Thanks again for the thoughtful feedback!