generate_packets.py: Move dsend fill into type code
Part of #45206. Make Field.get_fill() a wrapper around a call to a FieldType.get_code_fill() method, with new classes for different field types added as needed to provide different implementations for this method.
Part of #45206. Make Field.get_fill() a wrapper around a call to a FieldType.get_code_fill() method, with new classes for different field types added as needed to provide different implementations for this method.