Add an easier way to access Events API retry info

[seratch]

The issue #731 reminds me that https://github.com/slackapi/java-slack-sdk/pull/677 can be added to bolt-python (and possibly to bolt-js).

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

• [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
• [ ] Others

Requirements

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