[Umbrella] Support batch read for PK tables based on SST files + apply logs

### Search before asking

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

### Motivation

The community is actively working on [#1600 (FIP-17): Support Full KV Scan for Primary Key Tables](https://github.com/apache/fluss/issues/1600), which implements full PK table scanning via server-side RocksDB iteration. While this is a valuable direction, an alternative approach based on **downloading KV snapshot SST files to the client and applying incremental logs** remains meaningful for the following reasons:

1. **Server pressure:** FIP-17's approach requires the server to maintain scanner session context (`scannerId` / `callSeqId`) for each active scan. Under heavy concurrent query workloads, the server must manage many concurrent sessions, leading to significant memory and CPU overhead.
2. **Backward compatibility:** Many deployed server versions do not support Full KV Scan. The SST-file-based approach works against these older clusters without server-side changes.
3. **Compute pushdown:** Once SST files are downloaded to the client, local RocksDB can be leveraged for iteration, filtering, and projection, reducing server-side resource consumption.

Therefore, we propose supporting **both** approaches in Fluss, selectable via user configuration:
- **Server-side scan** (FIP-17): suitable for small tables and low-concurrency scenarios; lower latency
- **Client-side SST + log** (this proposal): suitable for large tables and high-concurrency scenarios; less server pressure

### Solution

```
┌──────────────────────────────────────────────────────────┐
│ Client Side │
│ │
│ ┌─────────────┐ ┌──────────────┐ │
│ │ KV Snapshot │ │ LogScanner │ │
│ │ SST Files │ │ (incremental)│ │
│ │ (download) │ │ (fetch) │ │
│ └──────┬──────┘ └──────┬───────┘ │
│ │ │ │
│ ▼ ▼ │
│ ┌─────────────┐ ┌──────────────┐ │
│ │SnapshotFiles│ │ Log Buffer │ │
│ │ Reader │ │ (RocksDB) │ │
│ │ (RocksDB) │ │ (dedup+sort) │ │
│ └──────┬──────┘ └──────┬───────┘ │
│ │ │ │
│ ▼ ▼ │
│ ┌──────────────────────────────────┐ │
│ │ SortMergeReader │ │
│ │ (snapshot + log merge by PK) │ │
│ └──────────────┬───────────────────┘ │
│ │ │
│ ▼ │
│ Sorted InternalRow stream │
└──────────────────────────────────────────────────────────┘
```

### Anything else?

_No response_

### Willingness to contribute

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