Add reason field to PullRequestEvent

[coding-CEO]

WHAT

• Add Reason *string field to PullRequestEvent struct
• Update tests to include new field in JSON marshaling
• Generate accessor method GetReason() with corresponding tests

WHY

• Field is populated in 'dequeued' event deliveries
• Fixes: reason is missing in the PullRequestEvent for dequeue action #3729

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.11%. Comparing base (1b53270) to head (874e9c1).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3730   +/-   ##
=======================================
  Coverage   91.11%   91.11%           
=======================================
  Files         187      187           
  Lines       16686    16686           
=======================================
  Hits        15204    15204           
  Misses       1295     1295           
  Partials      187      187           

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[gmlewis]

Thank you, @coding-CEO!
LGTM.
Merging.