gh-139653: Add PyUnstable_ThreadState_SetStackProtection() (#139668)

Add PyUnstable_ThreadState_SetStackProtection() and
PyUnstable_ThreadState_ResetStackProtection() functions
to set the stack base address and stack size of a Python
thread state.

Co-authored-by: Petr Viktorin <encukou@gmail.com>

Python/pystate.c

@@ -1495,6 +1495,9 @@ init_threadstate(_PyThreadStateImpl *_tstate,
     _tstate->c_stack_top = 0;
     _tstate->c_stack_hard_limit = 0;
 
+    _tstate->c_stack_init_base = 0;
+    _tstate->c_stack_init_top = 0;
+
     _tstate->asyncio_running_loop = NULL;
     _tstate->asyncio_running_task = NULL;