Add synonym types to bioentity {id} endpoints

[kshefchek]

See the request here - monarch-initiative/monarch-ui#334

Example: https://api-dev.monarchinitiative.org/api/bioentity/disease/MONDO:0007947

Currently bioentity/{type}/{id} combines all synonym types into one list, so the only way to get this is from the graph endpoint. We should make sure the pred field reflects the type of synonym, or break these out into different fields (the former probably makes more sense and won't break peoples code).