Move handler pipeline logic into internal/handlers package

Summary

I kinda don't like the app.go and the main package in generate right now. It does a lot of things and is a bit hard to follow. We can put all middleware configuration to a separate package and create a simple file for each of them.

• Also at least rename handlers.go to something related to artifacts. Yes it uses auth package, but it's just a workaround for authorizing these proxy requests. The main purpose of code in this file serving artifacts. A bit of refactoring this file may also be good 😉