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/commands/user.c

@@ -777,6 +777,8 @@ AlterRole(AlterRoleStmt *stmt)
 	/* Update indexes */
 	CatalogUpdateIndexes(pg_authid_rel, new_tuple);
 
+	InvokeObjectPostAlterHook(AuthIdRelationId, roleid, 0);
+
 	ReleaseSysCache(tuple);
 	heap_freetuple(new_tuple);
 
@@ -1161,6 +1163,8 @@ RenameRole(const char *oldname, const char *newname)
 
 	CatalogUpdateIndexes(rel, newtuple);
 
+	InvokeObjectPostAlterHook(AuthIdRelationId, roleid, 0);
+
 	ReleaseSysCache(oldtuple);
 
 	/*