gh-120642: Move private PyCode APIs to the internal C API (#120643)

* Move _Py_CODEUNIT and related functions to pycore_code.h.
* Move _Py_BackoffCounter to pycore_backoff.h.
* Move Include/cpython/optimizer.h content to pycore_optimizer.h.
* Remove Include/cpython/optimizer.h.
* Remove PyUnstable_Replace_Executor().

Rename functions:

* PyUnstable_GetExecutor() => _Py_GetExecutor()
* PyUnstable_GetOptimizer() => _Py_GetOptimizer()
* PyUnstable_SetOptimizer() => _Py_SetTier2Optimizer()
* PyUnstable_Optimizer_NewCounter() => _PyOptimizer_NewCounter()
* PyUnstable_Optimizer_NewUOpOptimizer() => _PyOptimizer_NewUOpOptimizer()