add "experiment type" or "study approach" key of some kind

[avanlinden]

Currently working off the "validation" vs "discovery" split for prioritizing AMP-AD 2.0 wrap up studies.

https://docs.google.com/document/d/1j8eFKFMHytyx65_egVHpiMSfum7WrBPaqd2j6Qk2MVI/edit