This Plugin will add a top-level featured image src field with the featured image src URL.
If you need to get a featured image from the WordPress REST API. This is a very simple plugin that will help.
Get the featured image without making an additional request, this will add direct access to the Featured Image URL.
The featured image Post type can be set in the admin section, where you can choose to enable or disable for specific post types.
REST API Featured Image Field: