RFR: 8260693: Provide the support for specifying a signer in keytool -genkeypair

[Hai-May Chao]

Please review the changes that adds the -signer option to keytool -genkeypair command. As key agreement algorithms do not have a signing algorithm, the specified signer's private key will be used to sign and generate a key agreement certificate.
CSR review is at: https://bugs.openjdk.java.net/browse/JDK-8264325

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Commit messages:
 - 8260693: Provide the support for specifying a signer in keytool -genkeypair

Changes: https://git.openjdk.java.net/jdk/pull/3281/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3281&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260693
  Stats: 521 lines in 6 files changed: 499 ins; 0 del; 22 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3281.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3281/head:pull/3281

PR: https://git.openjdk.java.net/jdk/pull/3281