#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

• 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

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• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
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• I've run ./scripts/install_all_and_run_tests.sh after making the changes.

[WilliamBergamin]

Thank you for your interest in our project and time spent contributing to it 🙏

I'm closing this PR since #1498 resolved #1484, for the time being we recommend using req.headers.get("x-slack-retry-num") and req.headers.get("x-slack-retry-reason") to access these properties

I will keep #732 open to keep track of users that absolutely want to access these values through the context