Add completion timestamp and historical listing to RebalanceProgress

### Search before asking

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

### Motivation

`Admin#listRebalanceProgress` returns `RebalanceProgress`, which carries `rebalanceId`, `status`, a `progress` fraction, and a per-bucket map — but no timestamp for when a rebalance started or finished. In addition, calling `listRebalanceProgress(null)` only ever returns the currently in-progress rebalance task; there's no way to retrieve information about rebalances that have already completed.

This makes it impossible to build any operator-facing tooling that answers "when did the cluster last rebalance?" or "how long did the last rebalance take?" — useful for health dashboards, alerting, and audit trails. Today that information is only recoverable by tailing server logs.

### Solution

- Add `startedAt`/`completedAt` (or equivalent) timestamp fields to `RebalanceProgress`.
- Retain a bounded history of recently-completed rebalance tasks (server-side or ZooKeeper-backed) so that `listRebalanceProgress` (or a new method) can return past tasks, not just the current in-progress one — including their final status (COMPLETED/FAILED/CANCELLED) and timestamps.
- A retention window/limit (e.g. keep the last N completed tasks) would bound the added storage/memory cost.

### Anything else?

_No response_

### Willingness to contribute

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

Start with Admin#listRebalanceProgress and RebalanceProgress, then trace how listRebalanceProgress(null) finds the current task and where rebalance state is stored. Define the API shape, timestamp semantics, completed-task retention limit, and final statuses before identifying the server-side or ZooKeeper-backed implementation. Done means current and bounded historical rebalances can be listed with timestamps and final status.

Written by the indexing model from the issue text.