public void addBuiltInFieldSetItem(String setName, String field) {
        builtInFieldSets.computeIfAbsent(setName, FieldSet::new).addFieldName(field);
    }
Adds an entry to built in field sets, creating entries as needed

@param setName name of a field set
@param field field to add to field set
config-model/src/main/java/com/yahoo/schema/FieldSets.java