Add cancel flag to AvaticaResultSet. The flag was added to CalciteResultSet as part of
CALCITE-1227, but would be useful in the Avatica base class.
It is currently of type CancelFlag but maybe it should become AtomicBoolean.
To complete implementation of java.sql.Statement#cancel we would need a Cancel command; that is out of scope of this case, and would be covered by CALCITE-520.