A system can potentially have 1 or more external IDs (i.e HBI_ID: $ID, SUBSCRIPTION_MANAGER_ID: $SUBMAN_ID) used for linking to an external system. We need to design a way to facilitate this. Given external IDs are crucial to deduplication, we should design changes for deduplication there as well.

We need to decide how this should be stored;  as a JSON, or separate table? Other?

QE tagged for review.