Fix #860 Enable developers to customize the built-in receivers more

[seratch]

Summary

This pull request adds a few customizable handlers to the default Receiver -- HTTPReceiver.

To: reviewers
I'm happy to elaborate more on details! but please refer to the comments in the diff first to learn the reason why these handlers can be useful.

Notes on other built-in receivers:

• SocketModeReceiver: This is a quite different one -- most of the changes are not applicable to this receiver
• ExpressReceiver: We can add some of these changes to this receiver but I personally would like to hold off adding the changes until we receive requests for the feature parity

Fixes #860

Requirements (place an x in each [ ])

• 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.

[seratch]

I've set the milestone to v3.9 but if other maintainers are fine, I'm happy to move this to v3.8 (the next release)

[codecov]

Codecov Report

Merging #1183 (9c2d8a7) into main (1b450f6) will increase coverage by 0.79%.
The diff coverage is 60.97%.

@@            Coverage Diff             @@
##             main    #1183      +/-   ##
==========================================
+ Coverage   71.42%   72.22%   +0.79%     
==========================================
  Files          17       17              
  Lines        1414     1433      +19     
  Branches      424      430       +6     
==========================================
+ Hits         1010     1035      +25     
+ Misses        332      322      -10     
- Partials       72       76       +4     

Impacted Files	Coverage Δ
src/receivers/HTTPReceiver.ts	44.82% <60.97%> (+7.73%)	⬆️

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[filmaj]

Looks good to me!

What sorts of documentation updates should go with this? Looking at the docs on Handling Errors, there is only mention of app.error in there - would that be a good place to add this functionality to? Another option would possibly be the Customizing Receiver docs? What do you think?

[seratch]

What sorts of documentation updates should go with this? Looking at the docs on Handling Errors, there is only mention of app.error in there - would that be a good place to add this functionality to? Another option would possibly be the Customizing Receiver docs? What do you think?

Yes, this is a really good point. We should update the error handling section. Also, we can update the reference page as well.

[filmaj]

Yes, this is a really good point. We should update the error handling section. Also, we can update the reference page as well.

OK, sounds good. I am happy to write those up if you wish.

[seratch]

OK, sounds good. I am happy to write those up if you wish.

Nice! Let's start with creating a task issue for it 👍

[filmaj]

Sounds good, created #1187 - if you'd like for me to work on it, please just assign me.

[seratch]

I've set the milestone to v3.9 but if other maintainers are fine, I'm happy to move this to v3.8 (the next release)

I'm thinking that we can have this in v3.8 next week. Before merging this, I wait for others' feedback / reviews by the end of next Modany, Pacific Time. Feel free to write in if you have any!

[stevengill]

Looks great Kaz!!