feat: make status code public in generated code

[karitham]

Fixes #84

[diamondburned]

I think we should also just export the other 2 fields while we're at it. Maybe even support io.ReadCloser for the Body.

[karitham]

Sourcing discord #goapi-gen for reference:

I feel like contentType should stay private, since it's kind of subject to change, and we might just add other headers / change how they behave
also, having the body as interface is kind of required since we have response to implement render, and thus the stuff can be marshalled as XML/YAML/JSON/whatever