[Protocol] Union support for row format

## Is your feature request related to a problem? Please describe.
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

## Describe the solution you'd like
A clear and concise description of what you want to happen.

## Additional context
#314

该 issue 未指明任何源文件、测试或入口点，只链接到 issue #314。首先阅读 #314，并追踪协议行格式设计，以确定预期的 union 行为。完成标准应包括有文档说明且经过测试的 union 表示形式，但此处未指定验收标准。

由索引模型根据 Issue 内容生成。