RFR: 8299414: JVMTI FollowReferences should support references from VirtualThread stack

[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;

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Commit messages:
 - tab
 - improved test
 - Merge branch 'openjdk:master' into vthread_follow_ref
 - update
 - tabs again
 - tabs
 - update
 - proto

Changes: https://git.openjdk.org/jdk/pull/13254/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13254&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299414
  Stats: 733 lines in 3 files changed: 636 ins; 86 del; 11 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