generate_packets.py: class for parsed field types
Part of #45206. Create a new class FieldType, which holds all information currently passed to the Field initializer in the typeinfo map. Move type parsing code to that class and use it to parse types once when first defined.
alienvalkyrie 
Part of #45206. Create a new class FieldType, which holds all information currently passed to the Field initializer in the typeinfo map. Move type parsing code to that class and use it to parse types once when first defined.