Add empty default operation to service operations

[jeaf-anaptecs]

Feature Description

When working with services it is currently not possible to add new operations to a serivce without having a breaking change.
Goal of this feature is to be able to extend existing services without having breaking changes in the code of the service provider.

Idea

• It is possible to mark a service operation that it should have a default implementation. Of course this default implementation does not have any business logic but at least it avoids that implementations have compile errors
• Concerning the implementation it should be possible to:
  • just do nothing / return null
  • throw a NotYetImplementedException(...)
  • return default value as defined in the UML model
  • execute a custom generator template that provides an implementation