RFR: 8372159: HttpClient SelectorManager thread could be a VirtualThread [v3]

[Daniel Fuchs]

> Each HttpClient instance creates an additional platform thread for its SelectorManager. With recent updates to NIO/VirtualThreads that thread could now become a VirtualThread. This would avoid having each HttpClient instance use up one platform thread.
> This is similar to what was done for the HttpClient QuicSelectorThread in [JDK-8369920](https://bugs.openjdk.org/browse/JDK-8369920).
> This should be transparent for users of the API. 
> An undocumented internal system property is introduced that can revert the change in case of unforeseen trouble.

Daniel Fuchs has updated the pull request incrementally with one additional commit since the last revision:

  Review feedback on test

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Changes:
  - all: https://git.openjdk.org/jdk/pull/28395/files
  - new: https://git.openjdk.org/jdk/pull/28395/files/dd4acad7..bdc84659

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28395&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28395&range=01-02

  Stats: 22 lines in 2 files changed: 18 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28395.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28395/head:pull/28395

PR: https://git.openjdk.org/jdk/pull/28395