Expose named object creation callback function at container level

For data model APIs:

`parent.add_on_child_created(child_type, cb)` ---> `named_object_container.add_on_object_created(cb)`.

And deprecate the former.

Search the data model API for the existing parent.add_on_child_created(child_type, cb) implementation and the named object container entry point. Trace how callbacks are registered and invoked, then expose named_object_container.add_on_object_created(cb) and mark the former API deprecated; done means both the new callback and deprecation behavior are covered by the relevant tests.

Written by the indexing model from the issue text.