Add ALTER OPERATOR command, for changing selectivity estimator functions.

Other options cannot be changed, as it's not totally clear if cached plans
would need to be invalidated if one of the other options change. Selectivity
estimator functions only change plan costs, not correctness of plans, so
those should be safe.

Original patch by Uriy Zhuravlev, heavily edited by me.

src/include/commands/defrem.h

@@ -73,6 +73,7 @@ extern void interpret_function_parameter_list(List *parameters,
 /* commands/operatorcmds.c */
 extern ObjectAddress DefineOperator(List *names, List *parameters);
 extern void RemoveOperatorById(Oid operOid);
+extern ObjectAddress AlterOperator(AlterOperatorStmt *stmt);
 
 /* commands/aggregatecmds.c */
 extern ObjectAddress DefineAggregate(List *name, List *args, bool oldstyle,