GH-107956: add sysconfig._build_details

This introduces a new sysconfig._build_details module to generate and
manipulate build-details.json data.

It contains abstractions to work with build-details.json data.
We hope that these APIs will evolve and reach the point it would make
sense to make them public.

In the future, we'd also like to abstract the data sources, so that the
code would be able to generate build-details.json for external
interpreters.

Signed-off-by: Filipe Laíns <lains@riseup.net>

Author: FFY00