bpo-38304: Add PyConfig.struct_size (GH-16451) (GH-16453)

bpo-38304: Add PyConfig.struct_size (GH-16451) (GH-16453)

Add a new struct_size field to PyPreConfig and PyConfig structures to
allow to modify these structures in the future without breaking the
backward compatibility.

* Replace private _config_version field with public struct_size field
  in PyPreConfig and PyConfig.
* Public PyPreConfig_InitIsolatedConfig() and
  PyPreConfig_InitPythonConfig()
  return type becomes PyStatus, instead of void.
* Internal _PyConfig_InitCompatConfig(),
  _PyPreConfig_InitCompatConfig(), _PyPreConfig_InitFromConfig(),
  _PyPreConfig_InitFromPreConfig() return type becomes PyStatus,
  instead of void.
* Remove _Py_CONFIG_VERSION
* Update the Initialization Configuration documentation.

(cherry picked from commit 441b10cf2855955c86565f8d59e72c2efc0f0a57)