[crac] RFR: 8378857: [CRaC] Expose checkpoint in CRaCMXBean

[Radim Vansa]

As checkpoint is practically a management operation it should be exposed and usable from the MXBean.

In fact the `JDK.checkpoint` operation is already exposed as DiagnosticCommand but this is a better well-known place.

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Commit messages:
 - 8378857: [CRaC] Expose checkpoint in CRaCMXBean

Changes: https://git.openjdk.org/crac/pull/298/files
  Webrev: https://webrevs.openjdk.org/?repo=crac&pr=298&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378857
  Stats: 105 lines in 7 files changed: 90 ins; 4 del; 11 mod
  Patch: https://git.openjdk.org/crac/pull/298.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/298/head:pull/298

PR: https://git.openjdk.org/crac/pull/298