Register and nest custom fields in Wordpress REST API (JSON)

Question

I would like to register custom fields to the Wordpress Rest API. So far I have the code at the bottom of this message, which works for registering one field.

How can I register/nest multiple custom fields under a single key?

I want my JSON to look like this, where everything under the "location" key is a custom field, returning those values from my custom field data:

 "location" : {
   "mapZoom": 
   "mapLat": 
   "mapLng": 
   "markerLat": 
   "markerLng": 
   "addressTitle":
   "address":
   "city": 
   "state": 
   "addressCountry":
   "geoJson": [ "mapLng", "mapLat" ]
},

Here's my code for registering single key / value pairs.

add_action('rest_api_init', 'register_custom_fields');
 function register_custom_fields(){
  register_rest_field(
    'post',
    'formatted',
        array(
        'get_callback' => 'show_fields'
    )
);
}

function show_fields($object, $field_name, $request){
return get_post_meta($object['id'], $field_name, true);
}