Add an endpoint that returns normalization results in SSSOM rather than JSON

[gaurav]

The Ontology Access Kit team is working on an adapter for this at INCATools/ontology-access-kit#397

I think this is pretty low priority, but I figured we should note this here and think about whether we want to offer such an endpoint in NodeNorm itself. One advantage of adding this directly to NodeNorm is that the translator won't know which of the conflated results were merged by conflation (unless it runs the query with and without conflation, which won't take too long actually).