Adding MergeGroupEvent

[Leolebleis]

Adding support for MergeGroup: https://docs.github.com/en/webhooks/webhook-events-and-payloads#merge_group

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.59%. Comparing base (2dab6f5) to head (e3e3371).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #254   +/-   ##
=========================================
  Coverage     77.59%   77.59%           
  Complexity      450      450           
=========================================
  Files            60       60           
  Lines          1446     1446           
  Branches         63       63           
=========================================
  Hits           1122     1122           
  Misses          279      279           
  Partials         45       45           

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[dennisgranath]

LGTM