Add the notion of REPLICA IDENTITY for a table.

Pending patches for logical replication will use this to determine
which columns of a tuple ought to be considered as its candidate key.

Andres Freund, with minor, mostly cosmetic adjustments by me

src/include/nodes/parsenodes.h

@@ -1284,9 +1284,17 @@ typedef enum AlterTableType
 	AT_DropInherit,				/* NO INHERIT parent */
 	AT_AddOf,					/* OF <type_name> */
 	AT_DropOf,					/* NOT OF */
+	AT_ReplicaIdentity,			/* REPLICA IDENTITY */
 	AT_GenericOptions			/* OPTIONS (...) */
 } AlterTableType;
 
+typedef struct ReplicaIdentityStmt
+{
+	NodeTag		type;
+	char		identity_type;
+	char	   *name;
+} ReplicaIdentityStmt;
+
 typedef struct AlterTableCmd	/* one subcommand of an ALTER TABLE */
 {
 	NodeTag		type;