bpo-36301: Add _Py_GetConfigsAsDict() function (GH-12540)

* Add _Py_GetConfigsAsDict() function to get all configurations as a
  dict.
* dump_config() of _testembed.c now dumps preconfig as a separated
  key: call _Py_GetConfigsAsDict().
* Make _PyMainInterpreterConfig_AsDict() private.