Follow-up from "Make `workflow:rules` work well with Merge Requests"

From @grzesiek: The amount of duplication between an entry/job and entry/bridge looks like something we should fix as a technical debt too. (out of scope here, of course).

From @ayufan: Yes, i agree. It is problematic to introduce concern today as a lot of configurable/validations works only in context of a given class, so we cannot easily deduplicate it yet.

We should maintain a single implementation across Config::Bridge and Config::Job, similar to how we do it with Ci::Processables.