[Test]: add test cases for Virtual Thread

[zhongkechen]

What would you like?

• Run some tests with Virtual Thread pool under Java 21+
• Automate the tests only for Java versions support VT
• Fix the potential bugs with VT under Java 21+

Possible Implementation

No response

Is this a breaking change?

No

Does this require an RFC?

No

Additional Context

No response

[zhongkechen]

Did some experiments in one branch:
https://github.com/aws/aws-durable-execution-sdk-java/tree/vt

PR:
#187

• Switched to using VT as the default thread pool and everything works fine.
• There is no performance advantages using VT
• VT can have higher concurrency, but not useful due to the 3000 operation limit.