gh-145779: Improve classmethod/staticmethod scaling in free-threaded build (#145826)

gh-145779: Improve classmethod/staticmethod scaling in free-threaded build (#145826)

Add special cases for classmethod and staticmethod descriptors in
_PyObject_GetMethodStackRef() to avoid calling tp_descr_get, which
avoids reference count contention on the bound method and underlying
callable. This improves scaling when calling classmethods and
staticmethods from multiple threads.

Also refactor method_vectorcall in classobject.c into a new _PyObject_VectorcallPrepend() helper so that it can be used by
PyObject_VectorcallMethod as well.