RFR: 8284926: Share the certificate NamedGroup in SignatureScheme::getSignerOfPreferableAlgorithm [v3]

[John Jiang]

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

John Jiang has updated the pull request incrementally with one additional commit since the last revision:

  add some comments

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8271/files
  - new: https://git.openjdk.java.net/jdk/pull/8271/files/eb706ae8..cd6e6dba

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8271&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8271&range=01-02

  Stats: 36 lines in 3 files changed: 9 ins; 12 del; 15 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