RFR: 8297878: KEM: Implementation [v12]

[Weijun Wang]

> The KEM API and DHKEM impl. Note that this PR uses new methods in https://github.com/openjdk/jdk/pull/13250.

Weijun Wang has updated the pull request incrementally with one additional commit since the last revision:

  more @since and about nulls

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13256/files
  - new: https://git.openjdk.org/jdk/pull/13256/files/8ba16065..51be3375

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13256&range=11
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13256&range=10-11

  Stats: 22 lines in 2 files changed: 10 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/13256.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13256/head:pull/13256

PR: https://git.openjdk.org/jdk/pull/13256