[Feature] Support to chain merge for the chain table

### Search before asking

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

### Motivation

Support to merge the serval delta partitions and snapshot into a new snapshot partition

### Solution

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

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

The issue names no files, tests, or entry points. Start by locating the chain-table implementation and existing snapshot or partition merge workflows, then clarify the intended merge semantics before defining what completion means.

Written by the indexing model from the issue text.