Add support for parsing code_scanning_alerts in webhooks

[emilva]

Following up on the issue I created https://github.com/google/go-github/issues/2391.

Let me know if there is anything missing or needs fixing. I just tried to follow the patterns from the other types.

Cheers,

emil

Fixes: #2391.

[codecov[bot]]

Codecov Report

Merging #2392 (1f8d04e) into master (8309da1) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2392   +/-   ##
=======================================
  Coverage   98.06%   98.06%           
=======================================
  Files         119      119           
  Lines       10546    10548    +2     
=======================================
+ Hits        10342    10344    +2     
  Misses        140      140           
  Partials       64       64           

Impacted Files	Coverage Δ
github/messages.go	100.00% <ø> (ø)
github/event.go	100.00% <100.00%> (ø)

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[emilva]

Thanks @gmlewis! :D

[gmlewis]

Thank you, @raynigon ! Merging.