Add a GetForeignUpperPaths callback function for FDWs.

This is basically like the just-added create_upper_paths_hook, but
control is funneled only to the FDW responsible for all the baserels
of the current query; so providing such a callback is much less likely
to add useless overhead than using the hook function is.

The documentation is a bit sketchy.  We'll likely want to improve it,
and/or adjust the call conventions, when we get some experience with
actually using this callback.  Hopefully somebody will find time to
experiment with it before 9.6 feature freeze.

src/include/foreign/fdwapi.h

@@ -59,6 +59,9 @@ typedef void (*GetForeignJoinPaths_function) (PlannerInfo *root,
 														  JoinType jointype,
 												   JoinPathExtraData *extra);
 
+typedef void (*GetForeignUpperPaths_function) (PlannerInfo *root,
+											   RelOptInfo *scan_join_rel);
+
 typedef void (*AddForeignUpdateTargets_function) (Query *parsetree,
 												   RangeTblEntry *target_rte,
 												   Relation target_relation);
@@ -166,6 +169,9 @@ typedef struct FdwRoutine
 	/* Functions for remote-join planning */
 	GetForeignJoinPaths_function GetForeignJoinPaths;
 
+	/* Functions for remote upper-relation (post scan/join) planning */
+	GetForeignUpperPaths_function GetForeignUpperPaths;
+
 	/* Functions for updating foreign tables */
 	AddForeignUpdateTargets_function AddForeignUpdateTargets;
 	PlanForeignModify_function PlanForeignModify;