Add metadata to mailable view data

[TobMoeller]

Background

While developing a package that filters outgoing emails by recipient using the MessageSending event, I noticed a discrepancy between notifications and mailables. Notifications include metadata like the notification class in the viewData (which is also provided to the event). However, when sending a Mailable, this functionality is not present and it is impossible to identify the originating class from the event.

What is currently present for notification context:

/**
 * Get additional meta-data to pass along with the view data.
 *
 * @param  \Illuminate\Notifications\Notification  $notification
 * @return array
 */
protected function additionalMessageData($notification)
{
    return [
        '__laravel_notification_id' => $notification->id,
        '__laravel_notification' => get_class($notification),
        '__laravel_notification_queued' => in_array(
            ShouldQueue::class,
            class_implements($notification)
        ),
    ];
}

Proposed Changes

This pull request introduces an additionalMessageData method to the Mailable class to enhance the provided viewData, mirroring the functionality present in the MailChannel class used for notifications. The aim is to provide consistent metadata in the MessageSending event across both notifications and mailables. As this is also passed to the MessageSending event, this enables event listeners to identify the message source.

Considerations

This PR does not fully mimic the notifications metadata. Currently i have only included the Mailable class to the meta data as I am not aware of a similar id attribute on mailables and checking for the implementation of the ShouldQueue interface might not be enough to identify if the mailable was queued (due to ways to queue a mailable without it's implementation). I might be wrong, but the "__laravel_notification_queued" information on notifications might also not be 100% accurate depending on the way it is queued, so it might be ok to include the same information for mailables for consistency?