Feature: #101700 - Use Symfony attribute to autoconfigure message handlers

Description

The symfony PHP attribute \Symfony\Component\Messenger\Attribute\AsMessageHandler is now respected and allows to register services as message handlers by setting the attribute on the class or the method.

Before:

EXT:my_extension/Configuration/Services.yaml

MyVendor\MyExtension\Queue\Handler\DemoHandler:
  tags:
    - name: 'messenger.message_handler'

After:

The registration can be removed from the Configuration/Services.yaml file and the attribute is assigned to the handler class instead:

EXT:my_extension/Classes/Queue/Handler.php

<?php

namespace MyVendor\MyExtension\Queue\Handler;

use MyVendor\MyExtension\Queue\Message\DemoMessage;
use Symfony\Component\Messenger\Attribute\AsMessageHandler;

#[AsMessageHandler]
final class DemoHandler
{
    public function __invoke(DemoMessage $message): void
    {
        // do something with $message
    }
}

It's also possible to set the attribute on the method:

EXT:my_extension/Classes/Queue/Handler.php

<?php

namespace MyVendor\MyExtension\Queue\Handler;

use MyVendor\MyExtension\Queue\Message\DemoMessage;
use Symfony\Component\Messenger\Attribute\AsMessageHandler;

final class DemoHandler
{
    #[AsMessageHandler]
    public function __invoke(DemoMessage $message): void
    {
        // do something with $message
    }
}

Impact

The registration of services as message handlers has been simplified by respecting the \Symfony\Component\Messenger\Attribute\AsMessageHandler attribute. When using this attribute, there is no need to register such service in the Configuration/Services.yaml file anymore. Existing configuration will work as before.