Feature/some refactorings

[n0900]

Fundamentally refactor ObjectBacked from interface with generics to abstract class without. In particular because ObjectBacked does no longer carry a generic type we should be able to use it in classes that need more than one serialization path (json and cbor for example) although this is future work

The previous generic (V) fixing the type of the backing object is not necessary. decode is renamed to getElement and original getElement removed. Integrated ObjectBackedValidated into ObjectBacked but the validate function has default implementation that does nothing.

The ultimate goal is that the helper classes such as JsonBackedObject are easy to implement and most logic lies in the abstract class extension functions (in ObjectBackedProperty.kt)