Add merge_group webhook event via MergeGroupEvent and MergeGroup structs

[lyoung-confluent]

Adds the structs for the new merge_group webhook event used by the Pull Request Merge Queue Limited Beta.

I'm not sure what the policy is on merging structs that are subject to change:

Note: The pull request merge queue feature is currently in limited public beta and subject to change.

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[codecov]

Codecov Report

Merging #2556 (6c695c1) into master (3bfe921) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2556   +/-   ##
=======================================
  Coverage   97.97%   97.98%           
=======================================
  Files         124      124           
  Lines       10840    10842    +2     
=======================================
+ Hits        10621    10623    +2     
  Misses        150      150           
  Partials       69       69           

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

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[gmlewis]

I'm not sure what the policy is on merging structs that are subject to change:

We tend to live on the cutting edge in this repo, so we have no problem with that, as the GitHub v3 API seems to be a continuously-moving target in my experience. That's not a criticism... they seem to be continuously innovating, and we're just trying our best to keep up. 😄

[gmlewis]

Thank you, @lyoung-confluent !
LGTM.

Awaiting second LGTM+Approval from any other contributor to this repo before merging.

[gmlewis]

Thank you, @valbeat !
Merged.