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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

:exclamation: 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

```diff @@ 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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