Add support for getting count in query

Ideally we want to be able to do a query and get the count of matching entries _without_ returning the data, just the count. This should be supported in FIQL queries. We would need to figure out the right Resource API for this (presumably has undesirable typing effect to have this an option for search directly, but a method that can do a query that returns empty objects or primary keys and counts them might be appropriate).