支持DB2

## Why you need it?
目前看AT模式下undo包下源码中是不支持DB2数据库？请问下这个是暂时没空支持还是说有什么技术难度（坑）吗？

## How it could be?
了解下没有支持的原因以便如果需要自己支持时候能少走弯路或者什么时候支持

## Other related information

The issue concerns DB2 support in Seata's AT mode and mentions the undo package source. Start by reviewing that source to determine the current database support boundary and the reasons DB2 is absent. Done would be a documented support assessment or a defined implementation path for adding DB2 support.

Written by the indexing model from the issue text.