Support for Broadcasting Streams about swiftmailer-postmark

[vladsandu]

@beberlei Thank you for submitting this PR and sorry for the delayed reply here.

As discussed in #20 , we want to defer most of this work to the API so that customers don't need to update their clients as frequently/when we add new functionality.

The Postmark API already supports setting the MessageStream via either the MessageStream field or the X-PM-Message-Stream header, like it's done for SMTP.

I have just tested this and can confirm that swiftmailer-postmark supports setting the MessageStream per message via:

$message->getHeaders()->addTextHeader('X-PM-Message-Stream', 'your-custom-stream');

The ability to set the MessageStream at transport-level might be useful for customers and we'll look over the PR and consider that as well, but you should already be able to set this at message-level.