Introduce value objects for auto save entries, split up ApiAutoSaveController::post

Overview

\Drupal\experience_builder\Controller\ApiAutoSaveController::post is doing a lot of juggling depending on the type of the autosave entry. 📌 Allow CMS Author to set site's homepage from navigation Postponed will add another case.
We should consider making the return type from the autosave manager value objects instead of associative arrays.
These could then hold the logic for each type e.g. -

* ContentEntityAutoSaveEntry
* ConfigEntityAutoSaveEntry
* PageTemplateAutoSaveEntry
* SimpleConfigAutoSaveEntry

Proposed resolution

User interface changes