[FEATURE]  Implement StreamAppned RPC

### Feature Description

<img width="1745" height="920" alt="Image" src="https://github.com/user-attachments/assets/66b25795-d4f4-4bfc-9237-0e6ce86ef56f" />

### Motivation

这是 Raft 流水线复制的核心 RPC，当前返回 unimplemented ，会导致 openraft 无法进行流水线优化。

### Additional Context

_No response_