Support workflow restrictions in actions_runner_groups

[bob-bins]

Fixes: #2558.

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[gmlewis]

Thank you, @bob-bins .
Please run "go generate ./..." and push the changes to the PR (we squash-and-merge all PRs) as described in CONTRIBUTING.md, and make sure to sign the CLA before we proceed.

[codecov]

Codecov Report

Merging #2559 (1f3d44e) into master (9598613) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2559   +/-   ##
=======================================
  Coverage   97.98%   97.98%           
=======================================
  Files         124      125    +1     
  Lines       10842    10875   +33     
=======================================
+ Hits        10623    10656   +33     
  Misses        150      150           
  Partials       69       69           

Impacted Files	Coverage Δ
github/actions_runner_groups.go	100.00% <ø> (ø)
github/github.go	97.37% <ø> (ø)
github/dependabot_alerts.go	100.00% <100.00%> (ø)

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[bob-bins]

@gmlewis I know the Contributors readme discourages force pushing, but I squashed and force-pushed when I corrected my commit signature. Thank you!

[gmlewis]

Thank you, @bob-bins !
Please make sure to run gofmt on the this file and its test file, and push the results after making the tweaks below.

[gmlewis]

Thank you, @bob-bins !
LGTM.
Merging.