Description
The mailchimp mailer is based on the mandrill API and create a payload
https://github.com/symfony/mailchimp-mailer/blob/5.x/Transport/MandrillApiTransport.php#L77-L83

But this payload could have other values
https://mandrillapp.com/api/docs/messages.html

For instance I'm using a lot track_opens and track_clicks.

$message = [
    'to'           => $recipients,
    'html'         => $body,
    'subject'      => $subject,
    'from_email'   => $fromEmail,
    'attachments'  => $attachments,
    'track_opens'  => true,
    'track_clicks' => true,
];

It would be great to add these two values (and maybe all the possible options of the API).

How should it be implemented ?
Does it require something like Email::getOptions() ?