Only create assertion class when it does not exist or when the associated class has changed

Currently, the generator plugin always generates the assertion class which requires the `test-compile` step of Maven to compile the classes in every run - even if no source code has changed. The assertion classes should only be generated:

1. when they do not exist
2. when the associated class changes

Example:

I want to create assertions for class `A`:

- assertion class for class `A` do not exist -> generate assertion class
- assertion class for class `A` exists and class `A` was not changed since the generation -> do not generate assertion class
- assertion class for class `A` exists and class `A` was changed since the generation -> generate a new version of the assertion class