[9.x] Add mailer name to data for SentMessage and MessageSending events

[colq2]

This PR adds the mailer name to data for the MessageSending and SentMessage events.

It is impossible to detect through which mailer a mailable is sent when listening to the MessageSending and SentMessage event.

There are some use cases where this is a benefit:

1. Stop sending the email depending on the mailer.

2. Storin outgoing mail with its mailer (from a package)

When using different mailers and a custom failover logic, it is important to know from which mailer a message was sent. Especially when using a package and the only way to intercept mails are the events.

Accessing the mailer from event data looks like this:

Event::listen(MessageSending::class, function(MessageSending $event){
    $mailer = $event->data['mailer'];
});