Generate id() method for EntityState types

[alexander-yevsyukov]

The EntityState interface defined in base has a generic parameter <I> but this parameter is not used in the interface (which is empty). We have a lengthy description of the convention for the first message field and so on, but it does not make it less confusing, “Why the generic parameter is not used in the interface?”

Obviously, having the id() method returning I would make it more obviously. But we need to tune ProtoData to handle this, and extend the interface accordingly.