Provide access to Delivery envelope in RabbitMQ Source

Currently RMQSource is limited to get AMQP message body in resulting Stream,

sometimes it would be good to get access to Envelope

For example 

if subscribing to Topic using Routing Key amq.topic.#

it is impossible to distinguish whether message came from amq.topic.1 or amq.topic.2

Studying current implementation I think the easiest option from consumer point of view would be to change RMQSource<OUT>  to provide Stream<RMQMessage<OUT>> which will break Flink API...

Another would be to create something like detailedsource with different API - provide generic Wrapper with message content <OUT> and Map with message properties (and protected method to override and fill that map).