Accept gzip encoded payloads

[josephjclark]

Description

This PR adds support for gzip-compressed payloads on webhooks.

Closes #3937

Validation steps

See issue

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[stuartc]

Will need to have some tests to see what happens when the body is malformed for gunzip. We will want a 4xx error and not a 500. But this implementation looks just fine to me.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.74%. Comparing base (fd4d39a) to head (44a618d).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/lightning_web/plug_configs.ex	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3938      +/-   ##
==========================================
- Coverage   88.77%   88.74%   -0.04%     
==========================================
  Files         422      422              
  Lines       18625    18629       +4     
==========================================
- Hits        16535    16532       -3     
- Misses       2090     2097       +7     

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[josephjclark]

@stuartc just done a big refactor to make it less shite, and generated some tests. Any thoughts? Can I toss it over the fence from here?