Add pass/blocklist for dataset types

[effigies]

It looks like the next BIDS release will have Study dataset types (bids-standard/bids-specification#1972) that allow a dataset to be nothing but a dataset_description.json (and possibly a bunch of ignored files). It would be good to allow users to reject these datasets if that doesn't match the goals of their application.

[yarikoptic]

loosely related to this a discussion started few days ago on "opt-in" validations and me thinking of some "tagging" system: https://groups.google.com/g/bids-discussion/c/taWbh968_Rs/m/nsdTUE6aAAAJ and validator potentially also being able to override the severity.

Here sounds similar on validator being able to perform differently on discovering some aspect. Additional aspects I might see worth "guarding for" and thus potentially taking into account while coming up with a specification here

• REQUIRE having sourcedata/raw if a "derivative" or "study" dataset
• FORBID having derivatives/ datasets

overall boiling down -- could it be somehow expressed as a collection of custom "schema rules" to be added for validator to be able to take?