AbstractValidatorChainEM2::attach compatible with parent class

[gszy]

Made the mentioned method’s signature (specifically, the $callback parameter) compatible with EventManager::attach() signature — added callable typehint.

[Xerkus]

Please note it is EventManager v2 compatiblity chain. https://github.com/zendframework/zend-eventmanager/blob/release-2.6.3/src/EventManager.php#L286

    /**
     * Attach a listener to an event
     *
     * The first argument is the event, and the next argument describes a
     * callback that will respond to that event. A CallbackHandler instance
     * describing the event listener combination will be returned.
     *
     * The last argument indicates a priority at which the event should be
     * executed. By default, this value is 1; however, you may set it for any
     * integer value. Higher values have higher priority (i.e., execute first).
     *
     * You can specify "*" for the event name. In such cases, the listener will
     * be triggered for every event.
     *
     * @param  string|array|ListenerAggregateInterface $event An event or array of event names. If a ListenerAggregateInterface, proxies to {@link attachAggregate()}.
     * @param  callable|int $callback If string $event provided, expects PHP callback; for a ListenerAggregateInterface $event, this will be the priority
     * @param  int $priority If provided, the priority at which to register the callable
     * @return CallbackHandler|mixed CallbackHandler if attaching callable (to allow later unsubscribe); mixed if attaching aggregate
     * @throws Exception\InvalidArgumentException
     */
    public function attach($event, $callback = null, $priority = 1)