[Feature] Support java api compaction of small files & Supports multi-threaded concurrent write？

### Search before asking

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

### Motivation

[Feature] Support java api compaction of small files & Supports multi-threaded concurrent write？

项目 https://github.com/melin/streamtunnel 是基于parquet，依赖计算引擎。纯java 生成parquet 文件，每15分钟一个分区，直接写文件，15分钟以后，合并小文件，再往hms 添加分区信息，用户才可以使用数据，后续考虑升级使用paimon，这样实时性就高了，也能支持cdc 数据。

如果同步大量表，计算引擎太耗资源，一般私有化场景承担不起那么多资源。

### Solution

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

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

No files, tests, or concrete API entry points are named. Start by locating the Java APIs for small-file compaction and concurrent writes, then clarify the expected behavior and constraints with maintainers. Done should mean that Java callers can trigger compaction and safely perform multi-threaded concurrent writes.

Written by the indexing model from the issue text.