…d (weaverryan)

This PR was merged into the 4.3-dev branch.

Discussion
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Dispatching two events when a message is sent & handled

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | none
| License       | MIT
| Doc PR        | TODO

Alternative to #30646. This uses a more generic system, so you could do anything when a message is sent. The main use-case is when a message is dispatched by a 3rd party.

I didn't try to add *exhaustive* events everywhere: I added an event for a very specific use-case:

When a message is dispatched by a 3rd party, being able to add stamps (e.g. `DelayStamp` or a future `AmqpRoutingKeyStamp` before the message is sent. Example:

```php
class MailerMessageSendToTransportEventSubscriber implements EventSubscriberInterface
{
    public function onSendMessage(SendMessageToTransportsEvent $event)
    {
        $envelope = $event->getEnvelope();
        if (!$envelope->getMessage() instanceof SomeMailerMessage) {
            return;
        }

        $event->setEnvelope($envelope->with(new AmpqRoutingKeyStamp('mailer-route')));
    }

    public static function getSubscribedEvents()
    {
        return [SendMessageToTransportsEvent::class => 'onSendMessage'];
    }
}
```

Along with #30557, we will now have the following events, regarding async messages:
* Event when a message is sent to transports (this PR)
* Event when a message is received from transport, but before handling it
* Event when a message is received from transport and after handling it

Commits
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a7ad1b4 Dispatching two events when a message is sent & handled