add support of offset lag

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.

### Motivation

Now we already have the timestamp lag in flink connector, but it's necessary to have offset lag especially for the long-term backfill

### Solution

_No response_

### Anything else?

_No response_

### Willingness to contribute

- [x] I'm willing to submit a PR!

Start by reading the Flink connector's existing timestamp-lag support and trace how lag is configured and applied. Define the corresponding offset-lag behavior for long-term backfills, then add coverage showing that records are selected using an offset lag and that the connector's existing timestamp-lag behavior remains intact.

Written by the indexing model from the issue text.