Add theadpool extension

[EvenLjj]

Background

SOFARPC provides the most basic thread pool implementation, which allows for setting thread pool related parameters and also supports customizing thread pools for specific interfaces. However, there is no extension provided to completely replace the original thread pool.

What we hope to achieve

• Define an extension interface for the thread pool and maintain the default implementation as before

[HzjNeverStop]

这个issue有进展吗？JDK21 开始支持了协程，基于这个能力应该能实现使用协程线程池替换掉 RPC 内置的业务线程池

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[EvenLjj]

#1383