Replace Sync AJAX callback with REST API endpoint.

[JakePT]

Description of the Change

Replaces the admin-ajax.php callbacks used by the Sync page with a REST API. This also adds support for additional arguments that are passed to the index helper:

• include
• indexables
• lower_limit_object_id
• offset
• post_type
• put_mapping
• upper_limit_object_id

This PR updates the Sync page to use the REST API, but there is no UI for the additional parameters at this stage.

Closes #3627

How to test the Change

• Sync should function as normal.
• POST requests to wp-json/elasticpress/v1/sync with the above parameters should perform the expected sync.
• GET requests to wp-json/elasticpress/v1/sync should return the status of an in progress sync.
• DELETE requests to wp-json/elasticpress/v1/sync should cancel an in progress sync.

Changelog Entry

Changed - Moved syncing from an admin-ajax.php callback to a custom REST API endpoint with support for additional arguments.

Credits

Props @JakePT

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[felipeelia]

@JakePT I added a small change that we will need. IMHO initializing the REST Sync class in Screen Sync seems out of place... now that we will have it available, that should be a standalone thing, so people who want to build on top of it would be able to (and not depend on any "screen" entity.) Thoughts?