Add amqp message dispatcher

[prolic]

I'll provide the adapter, when I have some time left.

[codeliner]

👍 that sounds great

FYI:
prooph/service-bus < v4.0 contained an interface for message dispatchers. Currently, all available message dispatchers still implement the interface because I had no time yet to adjust them.
Since v4.0 an interface is no longer required as a message dispatcher can use Prooph\Common\Messaging\MessageConverter to translate messages to plain PHP arrays independently.

My plan is to adjust the existing message dispatchers to be __invokeable. So a message dispatcher can act as a callable message handler and no further plugins are needed to integrate the message dispatcher.
In combination with the ServiceLocatorPlugin you are then able to lazy load a message dispatcher.

[prolic]

done with https://github.com/prooph/humus-amqp-producer