[FEATURE] Introduce Uniffle Server Flush Benchmark

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

### Search before asking

- [X] I have searched in the [issues](https://github.com/apache/incubator-uniffle/issues?q=is%3Aissue) and found no similar issues.

### Describe the feature

Supply a benchmark to tune and report the server flush performance.

### Motivation

Focus to test the flush performance.

### Describe the solution

_No response_

### Additional context

_No response_

### Are you willing to submit PR?

- [X] Yes I am willing to submit a PR!

The issue identifies the Uniffle server flush path as the benchmark target, but names no files, tests, metrics, or implementation approach. Start by locating the server's flush implementation and existing benchmark conventions, then confirm the expected workload and reporting format with maintainers. Done means a benchmark exercises flush performance and reports the agreed results.

Written by the indexing model from the issue text.