Move message handling types to abstractions projects

[stijnmoreels]

Is your feature request related to a problem? Please describe.
The Azure Service Bus message pump project contains several message handling abstractions that is now tightly-coupled to the message pump project. It would be good to extract these interfaces in a separate project.

Describe the solution you'd like

• Move the MessageHandling types from the Arcus.Messaging.Pumps.Abstractions to the Arcus.Messaging.Abstractions project/namespace
• Move the MessageHandling types from the Arcus.Messaging.Pumps.ServiceBus to the Arcus.Messaging.ServiceBus.Abstractions project/namespace.