Add EventDispatcher support to MailerService

[booooza]

• Adds an optional EventDispatcherInterface parameter to getMailer() method
• Maintains backward compatibility with existing code
• Enables listening to Symfony Mailer events (MessageEvent, SentMessageEvent, etc.)

This allows consuming packages to access debugging information like message IDs and transport debug data through Symfony's event system (see https://symfony.com/doc/current/mailer.html#debugging-emails).

For example you could define a simple event dispatcher that emits flow signals or execute any other logic.

<?php
declare(strict_types=1);

namespace Neos\SymfonyMailer\EventDispatcher;

use Neos\Flow\Annotations as Flow;
use Neos\Flow\SignalSlot\Dispatcher;
use Psr\EventDispatcher\EventDispatcherInterface;
use Symfony\Component\Mailer\Event\FailedMessageEvent;
use Symfony\Component\Mailer\Event\SentMessageEvent;

#[Flow\Scope("singleton")]
class FlowSignalEventDispatcher implements EventDispatcherInterface
{
    public function dispatch(object $event): object
    {
        if ($event instanceof SentMessageEvent) {
            $this->emitSentMessageEvent($event);
        } elseif ($event instanceof FailedMessageEvent) {
            $this->emitFailedMessageEvent($event);
        }

        return $event;
    }

    #[Flow\Signal]
    protected function emitSentMessageEvent(SentMessageEvent $event): void
    {}

    #[Flow\Signal]
    protected function emitFailedMessageEvent(FailedMessageEvent $event): void
    {}
}

If you think it makes sense, I could also include the above example as a default dispatcher with this PR.

[markusguenther]

Thank you very much. As this changes the API, although it is optional, I would see this change for a coming 1.1 release. I will test it soon and let you know.

But great that you came up with that change :)