code: public DruidQuerySignature asAggregateSignature(RowSignature sourceSignature)
    {
      return new DruidQuerySignature(
          sourceSignature,
          virtualColumnPrefix,
          virtualColumnsByExpression,
          virtualColumnsByName,
          true
      );
    }
docs: Create as an "immutable" "aggregate" signature for a grouping, so that post aggregations and having filters
  can not define new virtual columns
  @param sourceSignature
  @return