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 91.44%. Comparing base (06b8b3a) to head (723c401).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3774      +/-   ##
==========================================
+ Coverage   91.41%   91.44%   +0.03%     
==========================================
  Files         187      188       +1     
  Lines       16822    16900      +78     
==========================================
+ Hits        15377    15455      +78     
  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