Draft: Add prototype for internal output validation

The ReductionWriter creates an internal state about the output based on the reduction operations emitted, which the core can check against the actually written files. This is a draft how this could look like, implementing it only for remove-sources and remove-keys for now.

Must be turned via passing --validate.

@danilo Is this what you had in mind from a structural perspective?

mentioned in issue #4

This is indeed a good example of what I thought of. Of course, we can create also CI/CD pipelines in addition.

mentioned in issue #5