Add checks field to RequiredStatusChecks

[k24dizzle]

Attempt to solve #2274.

From my own experimentation, I discovered that there's two ways that this Checks field can be used to update the required status checks for a protected branch:

• update branch protection -> via the ProtectionRequest -> RequiredStatusChecks -> RequiredStatusCheck struct, in this API call we must provide either one of Checks or Contexts (the other must be nil) in RequiredStatusChecks to successfully update a branch protection. If you provide both or neither, you will get an Invalid Request error.
• update status check protection -> via the RequiredStatusChecksRequest -> RequiredStatusCheck struct, in this API call Checks and Contexts are both optional fields, if you provide both of them only Checks will be used to update the status checks.

[codecov]

Codecov Report

Merging #2276 (d801426) into master (51df45c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2276   +/-   ##
=======================================
  Coverage   97.81%   97.81%           
=======================================
  Files         114      114           
  Lines       10266    10266           
=======================================
  Hits        10042    10042           
  Misses        156      156           
  Partials       68       68           

Impacted Files	Coverage Δ
github/repos.go	98.70% <ø> (ø)

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[gmlewis]

Thank you, @k24dizzle !
Just a couple tweaks, please.

[gmlewis]

It looks like the linter has a couple issues as well.

[gmlewis]

Thank you, @k24dizzle !
LGTM.
Merging.