Add RequiredConversationResolution ProtectionRequest field

[orlovm]

For #1945 - added a new field for ProtectionRequest struct
to marshall as required_conversation_resolution JSON field
to API request for branch protection rules.

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[orlovm]

@googlebot I signed it!

[gmlewis]

Thank you, @orlovm !
LGTM.
Merging.

[codecov]

Codecov Report

Merging #1959 (57e3a08) into master (b338ce6) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1959   +/-   ##
=======================================
  Coverage   97.84%   97.84%           
=======================================
  Files         105      105           
  Lines        6809     6809           
=======================================
  Hits         6662     6662           
  Misses         80       80           
  Partials       67       67           

Impacted Files	Coverage Δ
github/repos.go	98.75% <ø> (ø)

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