fix: refactor function handler for follow up auto ack behavior

[WilliamBergamin]

Summary

This PR aims to

• Increase unit test coverage for the custom function handler
• Align the function handler with the middleware pattern used by actions and events
• Set up the project to allow simple introduction of auto acknowledgment flag

Manual tests

These changes were manually tested against

bolt-js-custom-step-template -> no breaking change observed 🟢
bolt-ts-starter-template -> no breaking change observed 🟢

Requirements

• 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.

[codecov]

Codecov Report

Attention: Patch coverage is 94.40000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 92.92%. Comparing base (82ab164) to head (8422fff).

Files with missing lines	Patch %	Lines
src/App.ts	89.09%	5 Missing and 1 partial ⚠️
src/CustomFunction.ts	98.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2424      +/-   ##
==========================================
+ Coverage   92.59%   92.92%   +0.32%     
==========================================
  Files          36       36              
  Lines        7472     7410      -62     
  Branches      653      653              
==========================================
- Hits         6919     6886      -33     
+ Misses        545      519      -26     
+ Partials        8        5       -3     

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