RFR: 8307378: Allow collectors to provide specific values for GC notifications' actions [v2]

[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.

William Kemper has updated the pull request incrementally with two additional commits since the last revision:

 - Fix missed implementation file
 - Unify message path

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13785/files
  - new: https://git.openjdk.org/jdk/pull/13785/files/d1792bb6..a85685b8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13785&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13785&range=00-01

  Stats: 77 lines in 16 files changed: 19 ins; 11 del; 47 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