Support for Broadcasting Streams

[beberlei]

The Swiftmailer transport for Postmark does not support broadcasting streams yet.

The method getMessagePayload needs to check for the X-PM-Message-Stream header defined in the docs to set the "streams" json key:

https://github.com/wildbit/swiftmailer-postmark/blob/master/src/Postmark/Transport.php#L167

https://postmarkapp.com/support/article/1207-how-to-create-and-send-through-message-streams

[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.