gh-76785: Improved Subinterpreters Compatibility with 3.12 (2/2) (gh-126707)

These changes makes it easier to backport the _interpreters, _interpqueues, and _interpchannels modules to Python 3.12.

This involves the following:

* add the _PyXI_GET_STATE() and _PyXI_GET_GLOBAL_STATE() macros
* add _PyXIData_lookup_context_t and _PyXIData_GetLookupContext()
* add _Py_xi_state_init() and _Py_xi_state_fini()