Extend object-access hook machinery to support post-alter events.

This also slightly widens the scope of what we support in terms of
post-create events.

KaiGai Kohei, with a few changes, mostly to the comments, by me

src/backend/rewrite/rewriteDefine.c

@@ -815,6 +815,9 @@ EnableDisableRule(Relation rel, const char *rulename,
 		changed = true;
 	}
 
+	InvokeObjectPostAlterHook(RewriteRelationId,
+							  HeapTupleGetOid(ruletup), 0);
+
 	heap_freetuple(ruletup);
 	heap_close(pg_rewrite_desc, RowExclusiveLock);