gh-99113: Add PyInterpreterConfig.own_gil (gh-104204)

We also add PyInterpreterState.ceval.own_gil to record if the interpreter actually has its own GIL.

Note that for now we don't actually respect own_gil; all interpreters still share the one GIL.  However, PyInterpreterState.ceval.own_gil does reflect PyInterpreterConfig.own_gil.  That lie is a temporary one that we will fix when the GIL really becomes per-interpreter.