Add ListExternalGroupsForTeamBySlug to Teams API

[bodgit]

#2217 added support for the majority of the External Groups API but omitted listing groups for a specific Team within an Organization.

This PR adds the missing method, and corrects the comment on the original ListExternalGroups method as that actually returns all external groups within the Organization. I added the ForTeamBySlug suffix based on other methods in the source to distinguish it from the original method.

I've added tests based on the original ListExternalGroups method and everything passes.

[codecov]

Codecov Report

Merging #2674 (1f336cc) into master (4814a50) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2674   +/-   ##
=======================================
  Coverage   98.05%   98.05%           
=======================================
  Files         131      131           
  Lines       11442    11455   +13     
=======================================
+ Hits        11219    11232   +13     
  Misses        152      152           
  Partials       71       71           

Impacted Files	Coverage Δ
github/teams.go	98.18% <100.00%> (+0.04%)	⬆️

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[gmlewis]

Thank you, @bodgit !
LGTM.

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

[bodgit]

@gmlewis good to merge now?

[gmlewis]

Thank you, @valbeat !
Merging.