MockAsWarGenerator: Allow subclasses to use an alternative MockAsWar

This PR introduces a new protected method that creates the MockAsWar instance. It allows user to create subclasses of the generator to use their own implementation of MockAsWar.
As a 1st use case, it will help me cleaning the code of the `mock-as-war` goal in [maven-soapui-extension-plugin](https://github.com/redfish4ktc/maven-soapui-extension-plugin)