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

[Alan Bateman]

This PR proposes to add a JDK-specific monitoring and management interface for the virtual thread scheduler. The interface is named VirtualThreadSchedulerMXBean and allows JMX based tooling monitor/manage the scheduler's target parallelism and monitor thread counts.

The JDK 5/6 era JDK-specific management interfaces are in jdk.management/com.sun.management. The proposal here is to start afresh in the jdk.management package, thus establishing a "new home" for JDK-specific management interfaces going forward.

jdk.test.lib.thread.VThreadRunner is test infrastructure used by several tests to set or ensure some level of parallelism. This is changed from using deep reflection to using the MXBean to access the target parallelism, which requires changes to the `@modules` tag tests in a number of tests (some tests no longer need to open java.base/java.lang).

(The changes proposed here have been in the loom repo for some time. A related command for the jcmd tool is also in that repo and will be proposed separately).

-------------

Commit messages:
 - Sync up from loom repo
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/20816/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20816&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338890
  Stats: 576 lines in 23 files changed: 523 ins; 17 del; 36 mod
  Patch: https://git.openjdk.org/jdk/pull/20816.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20816/head:pull/20816

PR: https://git.openjdk.org/jdk/pull/20816