[SendIt] Add custom header

[gam6itko]

Description

I need add some custom header to mail for testing purposes.
Something like:

X-Some-Secret-Code-From-Email: 123
X-New-User-Password: qwerty

• We could add new property $customHeaders to Mailer

• Pass it here.

• Handle it on mail render here

UPD it will be great to give user customize final email

For example add evend dispatch here.

$msg = new Email();

if ($message->getFrom() !== null) {
    $msg->from($message->getFrom());
}

$msg->to(...$message->getTo());
$msg->cc(...$message->getCC());
$msg->bcc(...$message->getBCC());

// new feature
$event = $this->eventDispatcher->dispatch(new PreRenderEvent(message: $message, email: $msg));

try {
    // render message partials
    $view->render(\array_merge(['_msg_' => $event->email], $event->message->getData()));
} catch (ViewException $e) {
}