RFR: 8248223: KeyAgreement spec update on multi-party key exchange support

[Jamil Nimeh]

This is a spec-only change that clarifies the requirement on KeyAgreement implementations for multi-party support where there are 3 or more parties involved.  This change states that 3+ party support is implementation-specific unless the underlying specification for that algorithm requires support for 3 or more parties.
Bug: https://bugs.openjdk.java.net/browse/JDK-8248223
CSR: https://bugs.openjdk.java.net/browse/JDK-8261456

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Commit messages:
 - 8248223: KeyAgreement spec update on multi-party key exchange support

Changes: https://git.openjdk.java.net/jdk/pull/2512/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2512&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8248223
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2512.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2512/head:pull/2512

PR: https://git.openjdk.java.net/jdk/pull/2512