EN-542 Add early access support for new MAPI endpoint `/early-access/…

[arguit]

…variants/filter`

Motivation

Which issue does this fix? Fixes #issue number

If no issue exists, what is the fix or new feature? Were there any reasons to fix/implement things that are not obvious?

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?

[huluvu21]

There might be some improvements possible, some comments should be improved and tests should cover (assert variant data) more.

[mattnield]

A question on this @pokornyd: are there any plans to allow us to use / access CompletionStatus for a specific content item/language variant rather than just a broader search across all variants that match the filter?