RFR: 8337199: Add jcmd Thread.vthread_scheduler and Thread.vthread_pollers diagnostic commands [v3]

[Alan Bateman]

> Adds `jcmd <pid> Thread.vthread_scheduler` to print the virtual thread scheduler and `jcmd <pid> Thread.vthread_pollers` to print the I/O pollers that support virtual threads doing blocking network I/O operations.
> 
> This is a subset of the diagnostics that we've had in the loom repo for a long time. @larry-cable proposed a PR recently ([pull/22121](https://github.com/openjdk/jdk/pull/22121)) to bring a version of same into main line but it was based on an older proposal. This new PR supplants that effort.
> 
> The jtreg failure handler is updated to execute Thread.vthread_scheduler, useful capture when a test fails or times out.

Alan Bateman has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Merge branch 'master' into JDK-8337199
 - VThread*Dcmd::execute changes to use helper function
 - Improve regex for matching poller String representation
 - Improve vthread_pollers output
 - Merge branch 'master' into JDK-8337199
 - Initial commit

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Changes:
  - all: https://git.openjdk.org/jdk/pull/22414/files
  - new: https://git.openjdk.org/jdk/pull/22414/files/2e9619a1..ea7df830

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22414&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22414&range=01-02

  Stats: 2120 lines in 75 files changed: 842 ins; 1052 del; 226 mod
  Patch: https://git.openjdk.org/jdk/pull/22414.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22414/head:pull/22414

PR: https://git.openjdk.org/jdk/pull/22414