gh-106320: Remove private _Py_Identifier API (#108593)

Remove the private _Py_Identifier type and related private functions
from the public C API:

* _PyObject_GetAttrId()
* _PyObject_LookupSpecialId()
* _PyObject_SetAttrId()
* _PyType_LookupId()
* _Py_IDENTIFIER()
* _Py_static_string()
* _Py_static_string_init()

Move them to the internal C API: add a new pycore_identifier.h header
file. No longer export these functions.