KCL-9696 - deliver js sdk timezone

[Radimersky]

Motivation

Time zone of date time element is now available in Deliver API. This PR adds the support of time zone for JS SDK. So when user uses this SDK to retrieve DateTime element, the element should contain new property "displayTimeZone". This property is either null or one of time zone ids (see DateTimeInItem: https://kontent.ai/learn/reference/delivery-api/#section/Custom-element).

Issue: KCL-9696

Parrent task: KCL-9687

Contact Marek Radiměřský if needed.

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

Current tests have been extended. No manual testing is required.

[Enngage]

LGTM, thanks!