Utilize inheritance where needed #64

Subject classes for build events do not have a proper parent.

For example

and https://github.com/cdevents/sdk-java/blob/5d17b750fda57fd4a9d7ad2c5baec2bb4b9e2143/sdk/src/main/java/dev/cdevents/models/build/started/Subject.java are very similar. These classes should have a parent class that handles the intersection between all subject object in all events. As in the type, id, source, etc. These should all live on a parent. This is an issue in various other places. I imagine Context has the same problem. We should definitely change how this behaves and how these classes are generated