Include result relation info in direct modify ForeignScan nodes.

FDWs that can perform an UPDATE/DELETE remotely using the "direct modify"
set of APIs need to access the ResultRelInfo of the target table. That's
currently available in EState.es_result_relation_info, but the next
commit will remove that field.

This commit adds a new resultRelation field in ForeignScan, to store the
target relation's RT index, and the corresponding ResultRelInfo in
ForeignScanState. The FDW's PlanDirectModify callback is expected to set
'resultRelation' along with 'operation'. The core code doesn't need them
for anything, they are for the convenience of FDW's Begin- and
IterateDirectModify callbacks.

Authors: Amit Langote, Etsuro Fujita
Discussion: https://www.postgresql.org/message-id/CA%2BHiwqGEmiib8FLiHMhKB%2BCH5dRgHSLc5N5wnvc4kym%2BZYpQEQ%40mail.gmail.com

src/include/nodes/execnodes.h

@@ -1777,6 +1777,7 @@ typedef struct ForeignScanState
 	ScanState	ss;				/* its first field is NodeTag */
 	ExprState  *fdw_recheck_quals;	/* original quals not in ss.ps.qual */
 	Size		pscan_len;		/* size of parallel coordination information */
+	ResultRelInfo *resultRelInfo;	/* result rel info, if UPDATE or DELETE */
 	/* use struct pointer to avoid including fdwapi.h here */
 	struct FdwRoutine *fdwroutine;
 	void	   *fdw_state;		/* foreign-data wrapper can keep state here */