generate_packets.py: Move handle args into type code
Part of #45206. Introduce a new Field.get_handle_arg() method, which calls a FieldType.get_code_handle_arg() method that passes a handle parameter as appropriate to match the declaration in get_code_handle_param() (see #45214).
Part of #45206. Introduce a new Field.get_handle_arg() method, which calls a FieldType.get_code_handle_arg() method that passes a handle parameter as appropriate to match the declaration in get_code_handle_param() (see #45214).