RFR: 8284926: Share the certificate NamedGroup in SignatureScheme::getSignerOfPreferableAlgorithm

[John Jiang]

It would not to generate the certificate's ECParameterSpec and NamedGroup multiple times in method `SignatureScheme::getSignerOfPreferableAlgorithm`.

-------------

Commit messages:
 - 8284926: Share the certificate NamedGroup in SignatureScheme::getSignerOfPreferableAlgorithm

Changes: https://git.openjdk.java.net/jdk/pull/8271/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8271&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284926
  Stats: 19 lines in 1 file changed: 5 ins; 8 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8271.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8271/head:pull/8271

PR: https://git.openjdk.java.net/jdk/pull/8271