Support sql_mode TIME_TRUNCATE_FRACTIONAL

[Thearas]

Feature Request

Is your feature request related to a problem? Please describe:
No.

Describe the feature you'd like:
Same as mysql TIME_TRUNCATE_FRACTIONAL. I don't want to round datetime, truncation is better in my scene.

Describe alternatives you've considered:
No.

Teachability, Documentation, Adoption, Migration Strategy:
See MySQL Doc: https://dev.mysql.com/doc/refman/8.0/en/sql-mode.html#sqlmode_time_truncate_fractional

[morgo]

Currently, TiDB behaves as if TIME_TRUNCATE_FRACTIONAL is always enabled. If this SQL mode is implemented, it will actually change the default behavior of TiDB to round, and require this mode to restore the current behavior.

Consider the following testcase:

CREATE TABLE t1 (a INT, b TIME(6));
INSERT INTO t1 (b) VALUES ('-10:10:59.500000');
INSERT INTO t1 (b) VALUES ('-10:10:59.500000');
UPDATE t1 SET a=b;
SELECT * FROM t1; 

..

tidb> SELECT * FROM t1; 
+---------+------------------+
| a       | b                |
+---------+------------------+
| -101059 | -10:10:59.500000 |
| -101059 | -10:10:59.500000 |
+---------+------------------+
2 rows in set (0.00 sec)

mysql [localhost:5735] {msandbox} (test) > SELECT * FROM t1; 
+---------+------------------+
| a       | b                |
+---------+------------------+
| -101100 | -10:10:59.500000 |
| -101100 | -10:10:59.500000 |
+---------+------------------+
2 rows in set (0.00 sec)