feat: Implement organization role assignments for both users and teams

[felixlut]

resolves#3280

[felixlut]

While working on this I noticed that the file naming is slightly off from how GitHub structures their API:s. There's a difference between repository roles and organization roles, but currently the orgs_custom_roles.go file is dealing with both (even before my changes). It should probably be split up into orgs_repo_roles.go and orgs_org_roles.go (or maybe flip the order to orgs_role_{org/repo}.go to keep the files "closer together"), similarily to integrations/terraform-provider-github#2224.

Leaving it up to the reviewer to decide if this should be done in this PR directly or in a separate one later

[gmlewis]

Leaving it up to the reviewer to decide if this should be done in this PR directly or in a separate one later

Please perform the reorganization in your next PR. Thank you!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.96%. Comparing base (2b8c7fa) to head (93c9317).
Report is 120 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #3281 +/- ##
==========================================
- Coverage 97.72% 92.96% -4.76% 
==========================================
Files 153 171 +18 Lines 13390 11705 -1685 ==========================================
- Hits 13085 10882 -2203 - Misses 215 729 +514 - Partials 90 94 +4 

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[gmlewis]

Thank you, @felixlut !
Just a few minor tweaks, please, then we should be ready for a second LGTM+Approval from any other contributor to this repo.

[felixlut]

@gmlewis done!

[gmlewis]

Thank you, @felixlut !
LGTM.

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

[tomfeigin]

LGTM

[gmlewis]

Thank you, @tomfeigin !
Merging.