Add annotation to allow types be declared as abstract or concrete

[christianhujer]

Regarding dependency management, the default behavior to treat technically abstract types such as interfaces, annotations, and abstract classes as abstract, and concrete classes, enums, and records as concrete is not always matching the effect that classes have on maintainability.

• Abstract classes and interfaces should not count as 100% abstract. They may define behavior, and there should be an option to determine their abstractness.
• POJOs and enums can be without behavior and thus be treated as abstract from a dependency management perspective.

Probably, the term abstract should go altogether and be replaced with the term stable.