Provide "migrate" helper to adopt (new) standardized aspects in datasets

[yarikoptic]

Your idea

Inspired by

• Allow for GeneratedBy in any sidecar .json file to collect their provenance #1970

I think it would be great to extract/formalize "migrate" functionality from #1775 and make it easy for users to convert already present fields into this new standardized form from earlier versions. I wonder if anyone knows already existing cases where some dataset(s) might be using some "pre-standard" metadata fields or files, which were later standardized in BEPs and thus could potentially be upgraded?

• may be for BEP028 GeneratedBy: "Absorb"/migrate already defined in BIDS dataset_description.json GeneratedBy BEP028_BIDSprov#148