[rebalance] Support rate‑limiting‑like effect by controlling number of partitions rebalanced at one time

### Search before asking

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

### Description

Support rate‑limiting‑like effect by controlling number of partitions rebalanced at one time

### Willingness to contribute

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

The issue provides no file, test, or entry point to begin from. First identify where partition rebalancing is implemented and how the project represents a rebalance batch; done means a configurable limit can produce the requested rate-limiting-like effect, with coverage for the resulting rebalance behavior.

Written by the indexing model from the issue text.