RFR: 8305310: Calculate PublicKey from PrivateKey

[Weijun Wang]

This code change is for DHKEM, which requires the function

pk(skX): The KEM public key corresponding to the KEM private key skX.

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Commit messages:
 - add a test
 - the fix

Changes: https://git.openjdk.org/jdk/pull/13250/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13250&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305310
  Stats: 184 lines in 7 files changed: 145 ins; 20 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/13250.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13250/head:pull/13250

PR: https://git.openjdk.org/jdk/pull/13250