Implement an API to let foreign-data wrappers actually be functional.

This commit provides the core code and documentation needed.  A contrib
module test case will follow shortly.

Shigeru Hanada, Jan Urbanski, Heikki Linnakangas

src/include/nodes/execnodes.h

@@ -1403,6 +1403,20 @@ typedef struct WorkTableScanState
 	RecursiveUnionState *rustate;
 } WorkTableScanState;
 
+/* ----------------
+ *	 ForeignScanState information
+ *
+ *		ForeignScan nodes are used to scan foreign-data tables.
+ * ----------------
+ */
+typedef struct ForeignScanState
+{
+	ScanState	ss;				/* its first field is NodeTag */
+	/* use struct pointer to avoid including fdwapi.h here */
+	struct FdwRoutine *fdwroutine;
+	void	   *fdw_state;		/* foreign-data wrapper can keep state here */
+} ForeignScanState;
+
 /* ----------------------------------------------------------------
  *				 Join State Information
  * ----------------------------------------------------------------