Is M3 NamespacedEntity an abstract concept or interface?

[enikao]

Pro abstract concept:
We once tended towards "classes have features, whereas interface exhibit behavior". In this sense NamespacedEntity feels like a class, as it has name and id.

Pro interface:
It feels closely related to NamespaceProvider and INamed, and both are interfaces.
Also, we didn't really follow through with above's semantics of classes vs. interfaces.

On 2023-06-30, we decided to make NamespacedEntity an interface.