Ability to attach macros to generated types and methods

It would be nice to be able to add macros to the generated Schema and API requests in order to extend the functionality, e.g. all of my request return structures have an optional error field, so if I could add a macro that would allow me to extend the support for a protocol for them, I could easily handle the error uniformly

No file, test, or entry point is named. Start by locating the generator code responsible for generated Schema types and API request return structures, then determine how macro attachment should be configured and represented; done means generated types and methods support the requested macros without breaking existing generation.

Written by the indexing model from the issue text.