Add the async module

Every @Async behaviour that surprises people, asserted by a test and captured
to docs/output/: the self-invocation trap, what CGLIB cannot override, the
IllegalArgumentException a plain return type throws, the unbounded queue that
makes max-size decoration, spring.task.execution.propagate-context (new in
Boot 4.1.0), the two Executor beans that leave @Async on an unpooled
SimpleAsyncTaskExecutor, and JEP 491 measured on JDK 21 against JDK 25.

async/docs/output/virtual-threads.txt

@@ -0,0 +1,10 @@
+== spring.threads.virtual.enabled=true, with pool properties still set ==
+
+applicationTaskExecutor : org.springframework.core.task.SimpleAsyncTaskExecutor
+@Async ran on           : task-1 (virtual=true)
+
+Executor beans in this context: 1
+  applicationTaskExecutor  ->  org.springframework.core.task.SimpleAsyncTaskExecutor  [TaskExecutor]
+Bean named 'taskExecutor' present: false
+Bean named 'applicationTaskExecutor' present: true
+