Show estimated number of groups for Incremental Sort in EXPLAIN

Show estimated number of groups for Incremental Sort in EXPLAIN

Incremental Sort's costs heavily depend on the estimated number of input
groups with equal presorted key values.  Overestimations can cause the
planner to choose Incremental Sort over Sort when Sort would have
been a better choice.

Here, we add the planner's estimate to EXPLAIN to allow easier
understanding of why Incremental Sort has been chosen.

Author: Ilia Evdokimov <ilya.evdokimov@tantorlabs.com>
Reviewed-by: Enrique Sánchez <enriqueesanchz@gmail.com>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Reviewed-by: solai v <solai.cdac@gmail.com>
Discussion: https://postgr.es/m/10682fef-3748-43f5-a932-7adcdd9bd2b8%40tantorlabs.com