This CL prepares WebSnapshot for skipping and re-injecting external
references in the web snapshot. External references are encoded as
separate object type and allows us to create partial snapshots at
runtime and reconnect a deserialised snapshot to an existing
object graph.

Part II will also collect all objects which cannot be serialized by the
web-snapshot serializer.

Usage:
  snapshot = %WebSnapshotSerialize(root, skip_externals);
  object = %eWebSnapshotDeserializ(snapshot, replaced_externals);

Drive-by-changes:
- Reduce JSObject Map size in serializer (we ended up with 4 embedder
  fields)
- Avoid adding non-HeapObject to the discovery_queue_
- Split off ReadXXX handlers into separate functions

Bug: v8:11525
Change-Id: Ia6a9914259614c6c288667621b38daa0202d4d72
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3461936Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79211}