[jdk17u-dev] RFR: 8364263: HttpClient: Improve encapsulation of ProxyServer

[Goetz Lindenmaier]

I backport this for parity with 17.0.18-oracle.

Clean backport, but needed to remove platform thread coding.

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Commit messages:
 - Adapt to old Syntax
 - backport e7cf18212b2dd75b9e068e5bb02e6372e8893954

Changes: https://git.openjdk.org/jdk17u-dev/pull/4111/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=4111&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364263
  Stats: 23 lines in 1 file changed: 5 ins; 10 del; 8 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/4111.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/4111/head:pull/4111

PR: https://git.openjdk.org/jdk17u-dev/pull/4111