RFR: 8369920: HttpClient QuicSelectorThread could be a VirtualThread

[Daniel Fuchs]

This change makes it possible to use a VirtualThread for the QuicSelectorThread. The default will be to use a VirtualThread on all platforms except the Windows platform. We might consider switching the default for Windows platform too once [JDK-8334574](https://bugs.openjdk.org/browse/JDK-8334574) is fixed.

The change should be transparent for users of the API. However, using a VirtualThread may result in subtle differences in thread scheduling and class loading, so this change also includes an escape hatch  (a non documented internal system property) that could be used to revert to a platform thread in case of unexpected issues.  That property may be removed in a future version of the JDK.

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Commit messages:
 - Add a new test
 - Merge branch 'master' into quic-vt-8369920
 - Merge branch 'master' into quic-vt-8369920
 - Draft - 8369920: HttpClient QuicSelectorThread could be a VirtualThread

Changes: https://git.openjdk.org/jdk/pull/27827/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27827&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369920
  Stats: 288 lines in 3 files changed: 277 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/27827.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27827/head:pull/27827

PR: https://git.openjdk.org/jdk/pull/27827