[jdk20u] RFR: 8307378: Allow collectors to provide specific values for GC notifications' actions

[William Kemper]

Unclean backport of JDK-8307378. Merge conflicts were trivial instances of `NULL`  vs `nullptr`.

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Commit messages:
 - Backport 1b143ba78712e7ac98ca9873c50989b3fba07394

Changes: https://git.openjdk.org/jdk20u/pull/75/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk20u&pr=75&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307378
  Stats: 88 lines in 19 files changed: 31 ins; 4 del; 53 mod
  Patch: https://git.openjdk.org/jdk20u/pull/75.diff
  Fetch: git fetch https://git.openjdk.org/jdk20u.git pull/75/head:pull/75

PR: https://git.openjdk.org/jdk20u/pull/75