WIP: Always tag omnibus and CNG together

For gitlab-com/gl-infra/delivery#794 (closed) we need to tag both cng and omnibus when either one of them changes. Unfortunately the logic for detecting changes and tagging was encapsulated which means we need to expose more to the caller and separate out the logic that detects a change and tags.

This is necessary because when we deploy to K8s we derive the CNG tag from the omnibus tag. The reason for this is because of the following pipeline chain:

[release tools] -- tags -> [cng]
[release tools] -- tags -> [omnibus] ---triggers with version--> [deployer] --triggers--> [kubernetes] which needs the CNG tag

@rspeicher before I go update the specs for this and clean it up do you think this is sane enough or should we think about how to go about this differently?