Fix #61 by correcting payload in middleware/listener args

[seratch]

This pull request fixes #61 by correcting the definitions of body and payload in middleware/listener args.

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

Requirements (place an x in each [ ])

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

• 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/run_tests.sh after making the changes.

[mwbrooks]

All tests pass ./scripts/run_tests.sh and nothing jumped out at me while reading through the changes.

This a huge refactor that touches a lot of code. As long as you're confident about the test coverage, then I think it's good to go!

[seratch]

Thank you for checking this! Yes, as we already have lots of unit tests including the ones for adapters, I'm sure that the changes do not break anything (actually in the development process, the tests helped me detect a lot of errors)

[seratch]

Also, I verified the changes not only by running tests but manually making sure if the samples work.

[seratch]

Ah, I noticed that a few samples need to be updated. I will do that before merging this.