Consider Stamping `DrugCentral:off_label` Edges with "Off Label" Edge Property

[ecwood]

Based on a discussion with Matt Brush, we should be indicating the type of use for drugs as an edge property. In particular, for off label uses, we should be using the ClinicalApprovalStatusEnum to indicate off label use. This applies to edges with the source relation DrugCentral:off_label use. It is not practical to do this in KG2 directly, so @saramsey was wondering if we could do this downstream.

Tagging @amykglen as requested by Steve