Post-V3 API Style Guide

[htdvisser]

Summary

I think it would be good if for our future API, assuming we'll still use protobuf, we start following a style guide based on the ones from Google and Buf.

Why do we need this?

In order to write a clean API that follows best practices (or at least: good practices that the ecosystem seems to be converging on) and allows us to generate properly working code in multiple languages, we should do less custom stuff and follow stricter guidelines.

What is missing? What do you want to see?

Some guidelines based on the style guides linked above. Probably also some guidelines related to what handwritten code (if any) belongs in the same (Go) packages as the generated code.