support mysql like start slave until

[azsane]

Feature Request

Is your feature request related to a problem? Please describe:
We have set up a replication between mysql and tidb using dm.
Some day,we found that a table was needed,but dm config didn't include it.

How do we enable this table in current version?
Through dmctl,we can update task config ,but TaskConfig is updated by units.
Mydumper unit's update method is empty.No new table data can be dumped
So we must import data manually.

And the problem is , we pause the task,import the new data,but we can't resume task
The position where we stopped and position we dumped the data may dismatch
There is no way to handle except another reimport

Describe the feature you'd like:
1.like mysql,add a command in dmctl to support stopping at binlog position/ gtid
then we can dump the data ,stop syncing at the exact position, importing data and finally resume task correctly.
2.if possible, support new table importing with update task command

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy: