Add support for organization Immutable Releases API

[Not-Dhananjay-Mishra]

Fixes: #3771.

This PR adds support for Organization Immutable Releases API

Implement

• GET /orgs/{org}/settings/immutable-releases
• PUT /orgs/{org}/settings/immutable-releases
• GET /orgs/{org}/settings/immutable-releases/repositories
• PUT /orgs/{org}/settings/immutable-releases/repositories
• PUT /orgs/{org}/settings/immutable-releases/repositories/{repository_id}
• DELETE /orgs/{org}/settings/immutable-releases/repositories/{repository_id}

issue - #3771

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.45%. Comparing base (06b8b3a) to head (3c5fcd9).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #3774 +/- ##
==========================================
+ Coverage 91.41% 91.45% +0.04% 
==========================================
Files 187 188 +1 Lines 16822 16904 +82 ==========================================
+ Hits 15377 15459 +82 
Misses 1257 1257 Partials 188 188 

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[gmlewis]

Thank you, @Not-Dhananjay-Mishra!
Just a few issues to address, please, then we should be ready for a second LGTM+Approval from any other contributor to this repo.

[Not-Dhananjay-Mishra]

@gmlewis and @zyfy29 Thanks for the suggestions, I’ve made some changes:
EnforcedRepositories string -> *string
SelectedRepositoriesIDs -> SelectedRepositoryIDs
in SetImmutableReleasesPolicy *ImmutableReleaseRepository -> ImmutableReleaseRepository
SetImmutableReleaseRepositories now accept directly repositoryIDs []int64 rather than SelectedRepositories struct
Made method descriptions in a single paragraph
and made a few other description tweaks

[gmlewis]

Thank you, @Not-Dhananjay-Mishra and @zyfy29!
LGTM.
Merging.