add ability to filter items based on presence or absence of entity attributes

Probably best implemented as an ItemSelectionStrategy implementation class or classes.

Basic case is selection if the entity has an entity attribute with literal matching of:

• format

• attribute

• value

Negation is probably required as well. Might be adequate to implement that as a second ItemSelectionStrategy class that negates the result. That might also be useful to implement and/or combinations although configuration will be very verbose.

Of course one could get into matching regular expressions, value ORing, and general attribute expressions but that's almost certainly not required for an initial pass.