RFR: 8357650: ThreadSnapshot to take snapshot of thread for thread dumps [v6]

[Alex Menkov]

> This is first (hotspot) part of the update for `HotSpotDiagnosticMXBean.dumpThreads` and `jcmd Thread.dump_to_file` to include lock information in thread dumps (JDK-8356870).
> The update has been split into parts to simplify reviewing.
> The fix contains an implementation of `jdk.internal.vm.ThreadSnapshot` class to gather required information about a thread.
> Second (dependent) part includes changes in `HotSpotDiagnosticMXBean.dumpThreads`/`jcmd Thread.dump_to_file`, spec updates and tests for the functionality.
> 
> Testing: new `HotSpotDiagnosticMXBean.dumpThreads`/`jcmd Thread.dump_to_file` functionality was tested in loom repo;
>   sanity tier1 (this fix only)

Alex Menkov has updated the pull request incrementally with one additional commit since the last revision:

  feedback

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Changes:
  - all: https://git.openjdk.org/jdk/pull/25425/files
  - new: https://git.openjdk.org/jdk/pull/25425/files/025d9f27..d1f463ba

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25425&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25425&range=04-05

  Stats: 20 lines in 3 files changed: 1 ins; 0 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/25425.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25425/head:pull/25425

PR: https://git.openjdk.org/jdk/pull/25425