[crac] RFR: 8347056: [CRaC] Propagating CRaC events to JVMTI agents

[Timofei Pushkin]

Adds JVM TI extension events corresponding to CRaC's beforeCheckpoint/afterRestore events.

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Commit messages:
 - Relax test requirement for extension event prefix
 - Propagating CRaC event to JVMTI agents

Changes: https://git.openjdk.org/crac/pull/172/files
  Webrev: https://webrevs.openjdk.org/?repo=crac&pr=172&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347056
  Stats: 211 lines in 10 files changed: 208 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/crac/pull/172.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/172/head:pull/172

PR: https://git.openjdk.org/crac/pull/172