Add @Generated annotation to generated code

[novalis]

Tools like https://github.com/google/error-prone use this to avoid running on generated code.

DKO's generated code fails the error-prone check because it is missing the @Override annotation for methods that override parent methods. Also, Errorprone flags some uses of == on Field objects, and maybe some other stuff.

Marking the code with javax.annotation.Generated will prevent this.