8338890: Add monitoring/management interface for the virtual thread scheduler

Reviewed-by: kevinw

src/java.base/share/classes/java/lang/VirtualThread.java

@@ -142,6 +142,14 @@ final class VirtualThread extends BaseVirtualThread {
     // termination object when joining, created lazily if needed
     private volatile CountDownLatch termination;
 
+
+    /**
+     * Returns the default scheduler.
+     */
+    static Executor defaultScheduler() {
+        return DEFAULT_SCHEDULER;
+    }
+
     /**
      * Returns the continuation scope used for virtual threads.
      */