RFR: 8366364: Return enabled signature schemes with SSLConfiguration#getSSLParameters() call [v7]

[Artur Barashev]

> For the signatureSchemes we return only what's been set by the user, the default values are not being returned like for other SSLParameters.
> Also enhancing "java -XshowSettings:security:tls" option to show the enabled signature schemes.

Artur Barashev has updated the pull request incrementally with one additional commit since the last revision:

  Add null custom value test case

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27961/files
  - new: https://git.openjdk.org/jdk/pull/27961/files/ef4877b9..691048f1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27961&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27961&range=05-06

  Stats: 25 lines in 1 file changed: 17 ins; 7 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/27961.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27961/head:pull/27961

PR: https://git.openjdk.org/jdk/pull/27961