[Bug] pd make the RaftEngine constructor private

### Bug Type (问题类型)

logic (逻辑设计问题)

### Before submit

- [X] 我已经确认现有的 [Issues](https://github.com/apache/hugegraph/issues) 与 [FAQ](https://hugegraph.apache.org/docs/guides/faq/) 中没有相同 / 重复问题 (I have confirmed and searched that there are no similar problems in the historical issue and documents)

### Environment (环境信息)

Server Version: 1.5

### Expected & Actual behavior (期望与实际表现)

The RaftEngine is intended to be a global singleton, but the constructor is not private.

```
public RaftEngine() {
this.stateMachine = new RaftStateMachine();
}
```

### Vertex/Edge example (问题点 / 边数据举例)

_No response_

### Schema [VertexLabel, EdgeLabel, IndexLabel] (元数据结构)

_No response_