syncer: support configure time_zone for downstream db

[lichunzhu]

Is your feature request related to a problem? Please describe

After pingcap/dm#1691 we deprecated time_zone variable for convenience.
However, sometimes we may need dm to execute dml in specific time_zone, for example, adding an extra column update_time datetime DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP in downstream table.
In this case, update_time will always be a UTC format time. However, we may want a CST time.

Describe the feature you'd like

Support configure time_zone variable in task.yaml. This variable only takes effect for syncer.

Describe alternatives you've considered

Change column update_time's type from datetime to timestamp. But timestamp may not be used after 2038.

Teachability, Documentation, Adoption, Migration Strategy

No response

[lance6716]

we can add it in session variables of config in my memory

[lichunzhu]

we can add it in session variables of config in my memory

We'll forcibly reset this now https://github.com/pingcap/dm/blob/83f62ae85d9d150f55c442dd99cf2dc82a49337a/dm/config/task.go#L934

[glorv]

In order to solve this issue, I think we can change our default time_zone from UTC to the global time_zone of downstream cluster. BTW, we will reserve the time_zone setting in task config in case there are other special requirements.

[lichunzhu]

In order to solve this issue, I think we can change our default time_zone from UTC to the global time_zone of downstream cluster. BTW, we will reserve the time_zone setting in task config in case there are other special requirements.

SGTM