[crac] RFR: Propagating CRaC event to JVMTI agents

[Roman Marchenko]

This change implements propagation of CRaC events to JVMTI.

- New JVMTI extension events are introduced - `EXT_EVENT_CRAC_BEFORE_CHECKPOINT` and `EXT_EVENT_CRAC_AFTER_RESTORE`.
- JVMTI agents should subscribe/unsubscribe to the events using `SetExtensionEventCallback` JVMTI method. 
  - Setting/unsetting a callback will enable/disable sending an appropriate event to an agent, i.e. there is no need to additionally call `SetEventNotificationMode` method by an agent.
  - See `test/jdk/jdk/crac/jvmtiEvents/libCracJvmtiAgent.c` for an example of CRaC-ready JVMTI agent

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Commit messages:
 - Added test
 - Propagated CRaC events to JVMTI agents

Changes: https://git.openjdk.org/crac/pull/143/files
 Webrev: https://webrevs.openjdk.org/?repo=crac&pr=143&range=00
  Stats: 212 lines in 9 files changed: 210 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/crac/pull/143.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/143/head:pull/143

PR: https://git.openjdk.org/crac/pull/143