fix: updating available event trigger data

[filmaj]

• removes app_id from message_posted event
• adds item_user, message_link and parent_message_link to reaction_added event
• removes inviter_id from user_joined_channel event

Our public docs on this are here: https://api.slack.com/automation/triggers/event#data

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (3d2507e) to head (15e7c8e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #114   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           36        36           
  Lines         1130      1133    +3     
  Branches        17        17           
=========================================
+ Hits          1130      1133    +3     

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[srajiang]

LGTM!