feat(alerting): Implement GitHub alerting provider

[TwiN]

Summary

Closes #378

Checklist

• Tested and/or added tests to validate that the changes work as intended, if applicable.
• Added the documentation in README.md, if applicable.

[codecov-commenter]

Codecov Report

Base: 82.04% // Head: 82.06% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (8c85ea7) compared to base (27502ac).
Patch coverage: 82.92% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #387      +/-   ##
==========================================
+ Coverage   82.04%   82.06%   +0.01%     
==========================================
  Files          54       55       +1     
  Lines        4066     4148      +82     
==========================================
+ Hits         3336     3404      +68     
- Misses        574      587      +13     
- Partials      156      157       +1     

Impacted Files	Coverage Δ
alerting/provider/provider.go	100.00% <ø> (ø)
alerting/provider/github/github.go	82.71% <82.71%> (ø)
config/config.go	73.26% <100.00%> (+0.14%)	⬆️

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