Add exists generated clause

[scirner22]

It has come up a few times that we needed the add-exists handler to produce a query clause that checks multiple values. Most notably if a resource is nested like /client/:client-id/resource/:id. Using (add-exists :id) allows an :id being returned that isn't associated with :client-id. The connections service uses two different ways to get around this. It has an input-transform that assoc's the :client-id into the key sweet-lib pulls from to generate the query and it also uses the :defaults sweet-lib key to assoc in a namespace to restrict queries for (/clients/:client-id/:namespace/connections). I think neither of these approaches are optimal and instead the add-exists handler should allow a variable number of arguments.