[Enhancement] How can I set the startup_mode to synchronize from the specified timestamp?

### Search before asking

- [X] I had searched in the [issues](https://github.com/apache/incubator-doris/issues?q=is%3Aissue) and found no similar issues.

### Description

I have a very large table and for some reason the synchronous redo log is overwritten, so i missing the kafka offset; the situation is that I can manually fix the data myself and I don t want to sync the historical data;
So how can I start syncing the data from the specified time point?

### Solution

_No response_

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

Start by reviewing the existing startup_mode behavior and how Kafka offsets are handled in the Doris Flink connector. Clarify how a specified timestamp should map to the synchronization start point, then verify that synchronization begins there without replaying earlier historical data.

Written by the indexing model from the issue text.