fix: add gitlab.SystemHookEvents to support Gitlab system events

[kamuridesu]

Closes #9625

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• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
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[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01 ⚠️

Comparison is base (e8f5126) 49.76% compared to head (eb3f6f0) 49.75%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12547      +/-   ##
==========================================
- Coverage   49.76%   49.75%   -0.01%     
==========================================
  Files         261      261              
  Lines       44653    44653              
==========================================
- Hits        22222    22219       -3     
- Misses      20248    20250       +2     
- Partials     2183     2184       +1     

Impacted Files	Coverage Δ
util/webhook/webhook.go	69.56% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[pasha-codefresh]

LGTM