[9.x] ability to add tags/metadata to Emails/Notifications

[kbond]

symfony/mailer has a special TagHeader and MetadataHeader for adding tags/metadata to emails in a generic way. All the transports/drivers you document support these with the slight exception of SES: it doesn't have tags in the way Symfony defines them but does have metadata. SES calls these tags so it's a bit confusing. Symfony's SES transport will support metadata in 6.1 but I see you created your own SESTransport. I can add support to your transport if desired.

Here's how tags and metadata are used with Postmark - all services are similar I believe.

I thought it would be nice to have helpers to add this data in Mailable/MailMessage:

// in your mailable:
public function build()
{
    return $this->from('example@example.com', 'Example')
                ->tag('shipment')
                ->metadata('order_id', $this->order->id)
                ->view('emails.orders.shipped');
}

// in your notification:
public function toMail($notifiable)
{
    return (new MailMessage)
                ->from('barrett@example.com', 'Barrett Blair')
                ->tag('upvote')
                ->metadata('comment_id', $this->comment->id)
                ->line('...');
}

If this is of interest, I can add tests/docs. Maybe you'd want the methods in a trait or done a different way - let me know!

[driesvints]

Cool, thanks @kbond!

[driesvints]

@taylorotwell I think this could indeed be useful for people now that we're integrating directly with Symfony Mailer.