feat(runtime)!: MCOL-678 A "GROUP BY ... WITH ROLLUP" support

Adds a special column which helps to differentiate data and rollups of
various depts and a simple logic to row aggregation to add processing of
subtotals.

primitives/primproc/batchprimitiveprocessor.cpp

@@ -1672,6 +1672,8 @@ void BatchPrimitiveProcessor::execute()
         {
           *serialized << (uint8_t)1;  // the "count this msg" var
 
+          // see TupleBPS::setFcnExpGroup2() and where it gets called.
+          // it sets fe2 there, on the other side of communication.
           RowGroup& toAggregate = (fe2 ? fe2Output : outputRG);
           // toAggregate.convertToInlineDataInPlace();