postgres_fdw: Fix cost estimation for aggregate pushdown.

In commit 7012b132d0, which added support for aggregate pushdown in
postgres_fdw, the expense of evaluating the final scan/join target
computed by make_group_input_target() was not accounted for at all in
costing aggregate pushdown paths with local statistics.  The right fix
for this would be to have a separate upper stage to adjust the final
scan/join relation (see comments for apply_scanjoin_target_to_paths());
but for now, fix by adding the tlist eval cost when costing aggregate
pushdown paths with local statistics.

Apply this to HEAD only to avoid destabilizing existing plan choices.

Author: Etsuro Fujita
Reviewed-By: Antonin Houska
Discussion: https://postgr.es/m/5C66A056.60007%40lab.ntt.co.jp

src/backend/optimizer/plan/planner.c

@@ -7113,6 +7113,10 @@ apply_scanjoin_target_to_paths(PlannerInfo *root,
 	 * confused in createplan.c if they don't agree.  We must do this now so
 	 * that any append paths made in the next part will use the correct
 	 * pathtarget (cf. create_append_path).
+	 *
+	 * Note that this is also necessary if GetForeignUpperPaths() gets called
+	 * on the final scan/join relation or on any of its children, since the
+	 * FDW might look at the rel's target to create ForeignPaths.
 	 */
 	rel->reltarget = llast_node(PathTarget, scanjoin_targets);