Add new callback 'scan_flags' for table access method

The table access method supports different features due to the different
implementation, this callback is used to indicate what the AM can do,
what features the AM can support.

Author: HuSen8891

src/include/access/tableam.h

@@ -390,6 +390,13 @@ typedef struct TableAmRoutine
 											  ScanDirection direction,
 											  TupleTableSlot *slot);
 
+	/*
+	 * This callback is used to indicate what the AM can do, what features the
+	 * AM can support, return the flags represented the supported features of
+	 * scan.
+	 */
+	int			(*scan_flags) (Relation rel);
+
 	/* ------------------------------------------------------------------------
 	 * Parallel table scan related functions.
 	 * ------------------------------------------------------------------------