Add orderby meta_value to WP_User_Query

[wpsmith]

For some reason, we cannot easily order the WP_User_Query() by last_name without directly editing the SQL statements. However, there is an easy way to add this that would work parallel to WP_Query().

// prepare arguments
$args  = array(
	'orderby' => 'meta_value',
	'meta_key' => 'last_name',
);
$user_query = new WP_User_Query( $args );

Related: #20606, #20624