Add DataObject.bindingToString()

mdsal-binding-java-api-generator's InterfaceTemplate needs to generate a

interface Foo extends DataObject {
     static @NonNull String bindingToString(final @NonNull Foo obj) {
         // ...
     }
}

method from the contents of generated builder implementation objects. Those need to defer to bindingToString() for the actual computation. The method needs to be static, so it can force its argument to also implement AugmentationHolder.