The ExternalPointerTags are assumed to be compile-time constants in most
cases, so turning them into template parameters enforces that. As
decisions such as whether to use the per-isolate or the shared external
pointer table are encoded into the tag values, forcing those to be
compile-time constants guarantees that the compiler will be able to
inline the correct logic when accessing an external pointer.

With this, there are now two (high-level) ways of accessing external pointer fields from C++: the Read/WriteExternalPointerField methods
which require the ExternalPointerTag to be a template parameter, and the
ExternalPointerSlot class which takes the tag as an argument. The latter
is for example used for snapshot deserialization and by the garbage
collector (more generally, by the ObjectVisitor::VisitExternalPointer
method), where the tag is not a compile-time constant.

Finally, this CL also introduces a new ExternalPointerHandle type which
represents the (opaque) on-heap representation of a reference to an
entry in an ExternalPointerTable when sandboxing is enabled. Making this
its own type makes the code a bit more readable.

Bug: v8:10391
Change-Id: I867b8ce41d15d485f1dc66786f233c710c56afcb
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/+/3720641Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81402}