This CL introduces a new ExternalPointerTable::Entry class and moves all
low-level logic related to entry management into this class.

Bug: v8:10391
Change-Id: Ib7eb05da1d277cb665503e98b3f074520e572bad
Cq-Include-Trybots: luci.v8.try:v8_linux64_heap_sandbox_dbg_ng,v8_linux_arm64_sim_heap_sandbox_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3829485Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82825}