Add enterprise runner group operations

[gabriel-samfira]

This change adds runner group operations on enterprises. The operations are essentially a mirror of the org runner group code, adapted for enterprises.

[codecov]

Codecov Report

Merging #2891 (428e724) into master (b700431) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2891      +/-   ##
==========================================
+ Coverage   98.12%   98.14%   +0.02%     
==========================================
  Files         142      143       +1     
  Lines       12453    12593     +140     
==========================================
+ Hits        12219    12359     +140     
  Misses        159      159              
  Partials       75       75              

Files Changed	Coverage Δ
github/enterprise_actions_runner_groups.go	100.00% <100.00%> (ø)

[gmlewis]

Thank you, @gabriel-samfira for the very clean and well-written PR!
❤️
Just one tiny nit, please, then we should be ready for a second LGTM+Approval from any other contributor to this repo before merging.

[gmlewis]

Thank you, @gabriel-samfira !
LGTM.

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

[gabriel-samfira]

I had forgotten about the omitempty tag. Added it now.

[WillAbides]

For consistency with ActionsService, should the "*RunnerGroup" methods be renamed to "*EnterpriseRunnerGroup"?

This PR also takes the opposite approach with the "*GroupRunners" methods. ActionsService doesn't include "Organization" in those method names, but EnterpriseService includes "Enterprise". I think the ActionsService names make more sense because they are acting on a "Runner", not an "OrganizationRunner" or "EnterpriseRunner".

[gmlewis]

For consistency with ActionsService, should the "*RunnerGroup" methods be renamed to "*EnterpriseRunnerGroup"?

This PR also takes the opposite approach with the "*GroupRunners" methods. ActionsService doesn't include "Organization" in those method names, but EnterpriseService includes "Enterprise". I think the ActionsService names make more sense because they are acting on a "Runner", not an "OrganizationRunner" or "EnterpriseRunner".

Excellent points, @WillAbides - I think you are right.
@gabriel-samfira - would you mind making these naming changes, please?

[gabriel-samfira]

Ohh wow. For some reason I did not get a notification for your comments. I will make these changes ASAP. Thanks for the review!

[gabriel-samfira]

Hopefully I got all the names right. Let me know if I missed (or misunderstood) anything.

[gmlewis]

Thank you, @WillAbides !
Merging.

[ajschmidt8]

Appreciate the work here! My team will benefit from these changes as well.

Thanks, everyone.