RFR: 8299414: JVMTI FollowReferences should support references from VirtualThread stack [v2]

[Alex Menkov]

> The fix updates JVMTI FollowReferences implementation to report references from virtual threads:
> - added heap scanning to report unmounted vthreads;
> - stacks of mounted vthreads are splitted into 2 parts (vittual thread stack and carrier thread stack), references are reported with correct thread id/class and object tags/frame depth;
> - common code to handle stack frames are moved into separate class;

Alex Menkov has updated the pull request incrementally with one additional commit since the last revision:

  aree -> are

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13254/files
  - new: https://git.openjdk.org/jdk/pull/13254/files/0eb9b050..8108f217

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13254&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13254&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13254.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13254/head:pull/13254

PR: https://git.openjdk.org/jdk/pull/13254