Refactor service into multiple crates and remove `#![allow(unused)]`

Currently, the integritee service contains far too much code. And with the introduction of the `link-binary` feature flag, there is much code, which is unused if we don't link it. I currently, disabled the linter warning, but ideally, we should refactor the code into several crates such that we can enable the lint again and it won't complain, as in this crate it is a public function.