RFR: 8307378: Allow collectors to provide specific values for GC notifications' actions

[William Kemper]

At the end of a GC pause, a `GarbageCollectionNotificationInfo` may be emitted. The notification has a `gcAction` field which presently originates from the field `_gc_end_message` in `GCMemoryManager`. Concurrent collectors such as Shenandoah, ZGC and G1 may have more (brief) pauses in their cycle than they have memory managers. This makes it difficult for gc notification listeners to determine the phase of the cycle that emitted the notification. We are proposing a change to allow collectors to define specific values for the `gcAction` to make it easier for notification listeners to classify the gc phase responsible for the notification.

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Commit messages:
 - Allow collectors to provide specific values for GC notification's action

Changes: https://git.openjdk.org/jdk/pull/13785/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13785&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307378
  Stats: 42 lines in 8 files changed: 19 ins; 0 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/13785.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13785/head:pull/13785

PR: https://git.openjdk.org/jdk/pull/13785