Function handler

[WilliamBergamin]

The goal of this PR is to move the change from my fork branch to a release candidate branch

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
• [ ] Document pages under /docs
• [ ] Others

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/install_all_and_run_tests.sh after making the changes.

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (4b086cf) 91.76% compared to head (fba35ae) 91.96%.

Files	Patch %	Lines
slack_bolt/request/internals.py	93.10%	2 Missing :warning:
slack_bolt/context/base_context.py	90.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## custom_function_release_candidate #1020 +/- ## ===================================================================== + Coverage 91.76% 91.96% +0.20% ===================================================================== Files 181 190 +9 Lines 6312 6509 +197 ===================================================================== + Hits 5792 5986 +194 - Misses 520 523 +3 ```

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