[feature](Nereids) add two functions: char and covert (#23104)
add [char](https://doris.apache.org/zh-CN/docs/dev/sql-manual/sql-functions/string-functions/char/?_highlight=char) func
```
mysql> select char(68, 111, 114, 105, 115);
+--------------------------------------+
| char('utf8', 68, 111, 114, 105, 115) |
+--------------------------------------+
| Doris |
+--------------------------------------+
```
convert func
```
MySQL root@127.0.0.1:(none)> select convert(1 using gbk);
+-------------------+
| convert(1, 'gbk') |
+-------------------+
| 1 |
+-------------------+
```
a136836770