Add support for filtering ESO datasets by proprietary

Right now it looks like the only way to check if an ESO dataset is proprietary is to check if `eso.retrieve_dataset` returns an empty list.

Start with the ESO entry point `eso.retrieve_dataset` and trace how dataset searches and empty results are handled. Determine how proprietary datasets can be identified and exposed as a filter; the work is done when callers can distinguish proprietary datasets without relying on an empty list.

Written by the indexing model from the issue text.