[Feature] Support reporting isProcessingBacklog as an input for checkpointing delay

### Search before asking

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

### Motivation

Flink 1.19+ support checkpointing delay during processing backlog by sources, https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=255069517, this is useful for accelerating the processing during full-stage of paimon scan, which may suffer from the checkpoint align.

### Solution

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

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

Start with the linked Flink 1.19+ checkpointing-delay documentation and trace Paimon’s Flink source and checkpointing integration; the issue names no files or tests. Determine how isProcessingBacklog should be reported during full-stage Paimon scans, and consider the work done when checkpointing delay can use that state.

Written by the indexing model from the issue text.