Add BranchProtectionRuleEvent Installation field

[nyarly]

Although it isn't yet documented at github, the BranchProtectionRuleEvent
includes an installation field which is crucial for letting Apps reacting to it
to authenticate their requests.

This PR adds that field the event type, along with an accessor for it.

Fixes: #2214.

[nyarly]

I've just added a CLA for this project.

[gmlewis]

@googlebot rescan

[gmlewis]

Thank you, @nyarly !
Please run go generate ./... and push (not force-push) the changed files to this PR, and the tests should then pass.
Hopefully @googlebot will also see your signed CLA by then.

[codecov]

Codecov Report

Merging #2216 (2f2e92d) into master (f262aa1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2216   +/-   ##
=======================================
  Coverage   97.81%   97.81%           
=======================================
  Files         112      112           
  Lines       10117    10117           
=======================================
  Hits         9896     9896           
  Misses        154      154           
  Partials       67       67           

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

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[gmlewis]

Thank you, @nyarly !
LGTM.

Merging.