This method now handles external pointers in HeapObjects during
serialization by encoding the representation of the external pointer
(sandboxed, raw), the origin (internal, api) and potentially the
external pointer tag. It is currently only used to handle
JSExternalObjects but could, in the future, be extended to handle all
external pointers that need special handling during
serialization/deserialization.

Bug: v8:12700
Change-Id: Ib0747d765ddc632e4ca4ee94521616d0271be0bc
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/+/3521904Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79496}