#732 Add an easier way to access Events API retry info

[siddik7181]

This PR adds an easier way to access Events API retry information within Bolt for Python apps by mirroring slackapi/java-slack-sdk#677 Addresses issue #732

Testing

Category

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

Requirements

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

• [x] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've run ./scripts/install_all_and_run_tests.sh after making the changes.