Add registered method change to sync stack server.
## Note
* We're keeping both generic and registered handler in generated stubs (`*_pb2_grpc.py`) because registered method flow is not implemented in aysncio yet, since the stub is used for both sync and asyncio, we can't remove generic method flow until we fully implement the registered method flow in asyncio.
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Closes#36371
PiperOrigin-RevId: 629842051
811bb258d8