Use the prototype Registry from either
DM-12613 or DM-12371 and the prototype Datastore from DM-12667 to implement a prototype Butler (as described in DMTN-056).
The only supported operations are Butler::get, Butler::put and Butler::unlink with a DatasetLabel (e.g. no querying, data-graphs or transfers).
The intent of this prototype is to inform design decisions for the final version. It should not be throw-away code but is allowed to be rough around the edges.