Ability to configure custom headers for webhooks

[stayallive]

Title is pretty descriptive here I think :)

Also removed the enableClientMessages in the webhook tests where client events are not used to prevent confusion.

[codecov]

Codecov Report

Merging #226 (6d52b02) into master (23ec401) will increase coverage by 0.17%.
The diff coverage is 100.00%.

❗ Current head 6d52b02 differs from pull request most recent head 1c9e894. Consider uploading reports for the commit 1c9e894 to get more accurate results

@@            Coverage Diff             @@
##           master     #226      +/-   ##
==========================================
+ Coverage   84.64%   84.81%   +0.17%     
==========================================
  Files          40       40              
  Lines        1830     1831       +1     
  Branches      323      324       +1     
==========================================
+ Hits         1549     1553       +4     
+ Misses        272      269       -3     
  Partials        9        9              

Impacted Files	Coverage Δ
src/app.ts	90.19% <ø> (ø)
src/webhook-sender.ts	85.00% <100.00%> (+0.18%)	⬆️
src/rate-limiters/cluster-rate-limiter.ts	96.87% <0.00%> (+9.37%)	⬆️

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