Provide a planner hook at a suitable place for creating upper-rel Paths.

In the initial revision of the upper-planner pathification work, the only
available way for an FDW or custom-scan provider to inject Paths
representing post-scan-join processing was to insert them during scan-level
GetForeignPaths or similar processing.  While that's not impossible, it'd
require quite a lot of duplicative processing to look forward and see if
the extension would be capable of implementing the whole query.  To improve
matters for custom-scan providers, provide a hook function at the point
where the core code is about to start filling in upperrel Paths.  At this
point Paths are available for the whole scan/join tree, which should reduce
the amount of redundant effort considerably.

(An alternative design that was suggested was to provide a separate hook
for each post-scan-join processing step, but that seems messy and not
clearly more useful.)

Following our time-honored tradition, there's no documentation for this
hook outside the source code.

As-is, this hook is only meant for custom scan providers, which we can't
assume very much about.  A followon patch will implement an FDW callback
to let FDWs do the same thing in a somewhat more structured fashion.

src/backend/optimizer/prep/prepjointree.c

@@ -909,6 +909,7 @@ pull_up_simple_subquery(PlannerInfo *root, Node *jtnode, RangeTblEntry *rte,
 	subroot->append_rel_list = NIL;
 	subroot->rowMarks = NIL;
 	memset(subroot->upper_rels, 0, sizeof(subroot->upper_rels));
+	memset(subroot->upper_targets, 0, sizeof(subroot->upper_targets));
 	subroot->processed_tlist = NIL;
 	subroot->grouping_map = NULL;
 	subroot->minmax_aggs = NIL;