gh-107954, PEP 741: Add PyConfig_Get()/Set() functions (#123472)

Add PyConfig_Get(), PyConfig_GetInt(), PyConfig_Set() and
PyConfig_Names() functions to get and set the current runtime Python
configuration.

Add visibility and "sys spec" to config and preconfig specifications.

_PyConfig_AsDict() now converts PyConfig.xoptions as a dictionary.

Co-authored-by: Bénédikt Tran <[email protected]>