Introduced APRDefaults to keep APR related name strings in one place

This is an attempt to create a single location where various APR-related constants/defaults/names can be defined in one place and accessed from all the places where they are used, both C++ and python.

The definition is done in C++ classes and propagated to python via a dictionary.
This MR contains an example use from both C++ and python - this is not a full migration yet but a proof of concept for a review.
Created as preparation for real work in ATEAM-933