Tag generated types with a marker interface

When we are generating a derived type, either as part of a leaf definition or a typedef, there is no marker interface identifying the fact that the type has a relationship with binding. This leads to our inability to provide services for such types in a type-safe manner.

Define a suitable yang-binding marker interface and make sure all GeneratedTypes carry it. Since GeneratedType is a superclass of GeneratedTransferObject, we need to make sure this tag is applied only to types which are not DataObjects – the reasoning being that DataObjects and these wrappers have different needs.

Also consider a 'BindingObject' type of interface, which would serve as the base for DataObject and this tagging interface.