Add support for External Groups

[kabesan]

Fixes: #2213 .
https://docs.github.com/en/enterprise-cloud@latest/rest/reference/teams#external-groups

[gmlewis]

Thank you, @kabesan !
Just a few tweaks, please, then we should be ready for a second LGTM before merging.

[gmlewis]

Also, please make sure to run unit tests before pushing (not force-pushing) your upcoming changes.
We squash and merge, so don't worry about the number of commits.
And finally, make sure to run go generate ./... and push those changes.
See our CONTRIBUTING.md file for more details.

[kabesan]

@gmlewis Thank your review.
I fiexed, Please re-review.

[codecov]

Codecov Report

Merging #2217 (c0a7d7d) into master (b26fa8f) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@ Coverage Diff @@## master #2217 +/- ##
==========================================
+ Coverage 97.79% 97.82% +0.02% 
==========================================
Files 112 112 Lines 10036 10166 +130 ==========================================
+ Hits 9815 9945 +130 
Misses 154 154 Partials 67 67 

Impacted Files	Coverage Δ
github/teams.go	98.14% <100.00%> (+0.20%)	⬆️
github/billing.go	100.00% <0.00%> (ø)
github/projects.go	96.32% <0.00%> (ø)
github/event_types.go	100.00% <0.00%> (ø)
github/repos_pages.go	97.11% <0.00%> (ø)
github/code-scanning.go	100.00% <0.00%> (ø)
github/actions_workflow_runs.go	100.00% <0.00%> (ø)
github/repos.go	98.70% <0.00%> (+0.01%)	⬆️
github/github.go	97.62% <0.00%> (+0.02%)	⬆️

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[gmlewis]

Thank you, @kabesan !
LGTM.

Awaiting second LGTM before merging.

(Please note that ALL other contributors to this repo are welcome to provide the second PR review/comment/approval that we need for merging and that we are not waiting for any particular reviewer unless otherwise noted.)

[kabesan]

Fixed for diff coverage!

[kabesan]

@gmlewis Hi
Is second reviewer not active?
Can you add other reviewers?

[gmlewis]

@gmlewis Hi Is second reviewer not active? Can you add other reviewers?

I am no longer able to add other reviewers, but any other contributor to this repo is welcome to review.

[kabesan]

Hi @pierce-m
Can you review this PR?
I'm waiting for second review.

[gmlewis]

Thank you, @pierce-m !
Merging.