[core] Add 5s timeout to the log and err subscriber polls.

[rynewang]

In worker.py we have threads to poll logs and errors from GCS. These polls are forever and we experienced some hang on shutdown. Changing them to a 5s timeout. This is safe because if it's timed out, PythonGcsSubscriber::DoPoll returns OK with empty error/log, and the python polling threads print_logs and listen_error_messages just ignores it and retries.

Also added some type annotations just for better code.

[rynewang]

This can fix #44729 but I think it should not hang in the first place, since we called worker.gcs_log_subscriber.close() right before joining the logger thread. We can merge this, while I will look a bit more into why it hung.