[crac] RFR: Propagating CRaC event to JVMTI agents [v3]

[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

Roman Marchenko has updated the pull request incrementally with one additional commit since the last revision:

  Fixed test

-------------

Changes:
  - all: https://git.openjdk.org/crac/pull/143/files
  - new: https://git.openjdk.org/crac/pull/143/files/0ad989c2..13c936b3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=crac&pr=143&range=02
 - incr: https://webrevs.openjdk.org/?repo=crac&pr=143&range=01-02

  Stats: 2 lines in 1 file changed: 0 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