expose event handlers for receiving different message types

[oliverrahner]

Is your feature request related to a problem? Please describe.
It is kind of hard for developers to understand what kinds of messages they can receive and what they need to handle.

Describe the solution you'd like
We could provide a deeper level of events, one for each message type (https://github.com/DKE-Data/messageadapter/blob/088fc103e2a85d931a31aced49dddea895a69fea/ma-ar-router/src/proto/response.proto#L12).
This could be detailed even deeper, e.g. by providing handlers like IsoxmlFileReceived() as a deeper level for MessageReceived().
In the same way, Push Notifications can already be decoded and provided to those lower level event handlers.

Additional context
Thanks to @alexfangmann for the suggestion