A delegate that a client can register that will be called each time a consumer dispatches a message
to the client code to allow the client to Transform a received message from one type to another,
StreamMessage to TextMessage, ObjectMessage to TextMessage containing XML, etc. This allows a
client to create a consumer that will automatically transform a message to a type that the client is
capable of processing or adding additional information to a received message. For messages that do
not need to be processed the client should return null from this method, in this case the original
message will be dispatched to the client.
function(session, consumer, message);