建议AT模式加一个二阶段分支事务回滚和提交的扩展点，全局提交或者回滚之后需要做一些操作

我这边是有这么一个场景，在分布式事务的分支事务中，有一个异步消息通知的功能，分支事务提交后发送了消息；如果后续全局事务回滚，分支事务也进行了回滚，但是消息已经发出去了，没法回滚。建议加一个全局事务提交或者回滚的listener，只有在全局事务提交后，我在做消息发送，保证消息的真实性。

The issue names no files, tests, or entry points. Start by tracing Seata's AT transaction lifecycle and how branch commits and rollbacks are coordinated globally. Done would mean a listener or extension point can send notifications only after the global transaction commits, or react after it rolls back, without publishing messages from an individual branch commit.

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