`TIMESTAMPADD` does not return correct data type when parameter is DateTime type

[birdstorm]

Enhancement

This issue is found during #38003

TIMESTAMPADD should return different DataTypes according to the DataType of its parameters.

e.g.,
MySQL behavior:

mysql> create view v(result) as select timestampadd(second,1.1,cast('1995-01-05 06:32:20.859724' as datetime)) as result;
Query OK, 0 rows affected (0.01 sec)

mysql> desc v;
+--------+-------------+------+-----+---------+-------+
| Field  | Type        | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| result | datetime(1) | YES  |     | NULL    |       |
+--------+-------------+------+-----+---------+-------+
1 row in set (0.01 sec)

mysql> select * from v;
+-----------------------+
| result                |
+-----------------------+
| 1995-01-05 06:32:22.1 |
+-----------------------+
1 row in set (0.00 sec)

TiDB behavior:

mysql> create view v(result) as select timestampadd(second,1.1,cast('1995-01-05 06:32:20.859724' as datetime)) as result;
Query OK, 0 rows affected (0.12 sec)

mysql> desc v;
+--------+-------------+------+------+---------+-------+
| Field  | Type        | Null | Key  | Default | Extra |
+--------+-------------+------+------+---------+-------+
| result | varchar(19) | NO   |      | NULL    |       |
+--------+-------------+------+------+---------+-------+
1 row in set (0.01 sec)

mysql> select * from v;
+---------------------+
| result              |
+---------------------+
| 1995-01-05 06:32:22 |
+---------------------+
1 row in set (0.00 sec)

When using TIMESTAMPADD, TiDB always casts the parameter into String, and always returns a String result. This is not efficient and in-compatible with MySQL behavior.

[windtalker]

Change the type to bug since always return string will hurt the performance.