Adding Orderby support for custom meta fields

function.php

add_filter(
    'rest_' . $post_type . '_query',
    function ( $args, $request ) {
		 $fields = ["first_name","last_name","address","email"];
        $order_by = $request->get_param( 'orderby' );
        if ( isset( $order_by ) && in_array($order_by, $fields)) {
            $args['meta_key'] = $order_by;
            $args['orderby']  = 'meta_value'; // user 'meta_value_num' for numerical fields
        }
        return $args;
    },
    10,
    2
);