ProtectedBranches API handles per user/group granularity

What

Allow protected branch access levels to be set per user or group and includes user_id/group_id in GET requests

Why

Granular control over individual users/groups was added to the UI but is not yet mirrored in the API

Acceptance criteria

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Backend
• Internationalization required/considered

Related

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/4131