UnionAll should be able to reserve the order property comming from its children

## Enhancement

suppose that we have table
```
create table t(a int, index idx(a));
```

and a SQL `select * from (select a from t union all select a from t) t order by a limit 1`.

The best plan is that we keep the order property by reading index `idx` of table t and the union can also use the order property to perform a merge sort to output the data. In this way, we don't need any extra sort to sort the data can directly perform limit upon the union all.

current TiDB lacks the functionality and will do a sort after the union all. no rewrite can achieve the best possible plan until the union all can do the merge sort.