generate_packets.py: Move field get into type code
Part of #45206. Make Field.get_get_real() a wrapper around a call to FieldType.get_code_get() that reads field values as appropriate.
This patch also passively fixes an issue where diff arrays weren't working correctly in JSON, non-delta mode.
Part of #45206. Make Field.get_get_real() a wrapper around a call to FieldType.get_code_get() that reads field values as appropriate.