Support metakg queries with biolink model's semantic descendants #185
Description
Each edge in SmartAPI's metakg contains subject, object, predicate fields which are the terms based on the biolink model. We should add the metakg query support for searching on either of these fields by optionally expanding to include all descendant terms.
We can use biolink-model-toolkit to get the descendant of a given term.
In metakg's query handler, we can add explicit subject, object and predicate query parameters, and an optional expand parameter when can take values of subject, object and predicate (include their combinations), so that we can expand either or multiple fields when needed. Default is still not expanding these terms.