Use `plz generate` for code generation

[VJftw]

Enhancement

Current behaviour

Our generated code only lives in plz-out

Disadvantages of current behaviour

We do not get editor support/code completion for protobuf libraries in our IDEs as they are generated code.

New behaviour

We use the new plz generate code feature which copies outputs of build rules outside of plz-out appropriately. We can utilise .gitignore files to prevent them being committed to the repository where desired.

Advantages of new behaviour

• IDE support
• Generated Go protobuf files can be used externally