<?php

function my_rest_prepare_post( $data, $post, $request ) {

	$_data = $data->data;

	$thumbnail_id = get_post_thumbnail_id( $post->ID );
	$thumbnail = wp_get_attachment_image_src( $thumbnail_id );
	$_data['featured_image'] = $thumbnail[0];
	
	$data->data = $_data;
	return $data;
}
add_filter( 'rest_prepare_post', 'my_rest_prepare_post', 10, 3 );