Lack of interest Define a public interface for \XF\Mail\Queue

[Sim]

If we want to swap out the Mail queue, it would be useful to have the public interface that the Mailer is going to use defined.

For example:

<?php

namespace XF\Mail;

interface MailQueue
{
    public function queue(\Swift_Mime_Message $message);
   
    public function queueForRetry(\Swift_Mime_Message $message, $queueEntry);
}

... and then the Queue class would become:

<?php

namespace XF\Mail;

class Queue implements MailQueue
{
    ...
}

... and the constructor for the Mailer would typehint the XF\Mail\MailQueue interface rather than the XF\Mail\Queue class.