EN-130 - Add schedules to content item variant

[VladimirO-kontent]

Motivation

Adds readonly schedule object to content item variant, scheduled publish/unpublish times and display timezones.

Checklist

• Code follows coding conventions held in this repo
• Automated tests have been added
• Tests are passing
• Docs have been updated (if applicable)
• Temporary settings (e.g. variables used during development and testing) have been reverted to defaults

How to test

If manual testing is required, what are the steps?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.34%. Comparing base (24e60b5) to head (1110bce).
Report is 2 commits behind head on master.

❗ Current head 1110bce differs from pull request most recent head 6f04622. Consider uploading reports for the commit 6f04622 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #261      +/-   ##
==========================================
+ Coverage   85.30%   85.34%   +0.03%     
==========================================
  Files         267      268       +1     
  Lines        2927     2934       +7     
  Branches      318      318              
==========================================
+ Hits         2497     2504       +7     
  Misses        337      337              
  Partials       93       93              

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