Add a new FE Config `sql_convertor_service`.
If this config is set, and the session variable `sql_dialect` is set,
Doris will try to use a standalone sql converter service to convert user input sql to
specified sql dialect. eg:
```
mysql> set sql_dialect="presto";
Query OK, 0 rows affected (0.02 sec)
Database changed
mysql> select * from db1.tbl1 where "k1" = 1; # will be converted to select * from db1.tbl1 where `k1` = 1;
+------+------+
| k1 | k2 |
+------+------+
| 1 | 2 |
+------+------+
1 row in set (0.08 sec)
```
The sql converter service should be a http service.
The request and response body can be found in `SQLDialectUtils.java`
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