src: allow building the snapshot on top of a V8 startup blob

CommonEnvironmentSetup::CreateForSnapshotting() always created the
SnapshotCreator without a startup blob, so V8 set up its heap from
scratch. That is impossible when the V8 that Node.js is linked against
only carries the deserializer (external startup data, as Chromium builds
it), and it puts the resulting snapshot on a different read-only heap
lineage than isolates the embedder creates from its own blob. Embedders
in that situation cannot use the snapshot support at all today.

Add SnapshotConfig::base_blob, an optional caller-owned v8::StartupData
that is handed to the SnapshotCreator so that V8 deserializes its heap
from that blob and Node.js adds its isolate data and contexts on top,
the way Blink's context snapshot is built. node_mksnapshot accepts
--v8-snapshot-blob=<file> for hosts that build Node.js with such a V8;
Node.js's own build never passes it and is unchanged. Consuming the
result needs no changes.

embedtest can create a plain V8 startup blob and build the embedder
snapshot on top of one, and a test round-trips argv through a snapshot
built that way.

Signed-off-by: Shelley Vohr <shelley.vohr@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/65374
Reviewed-By: James M Snell <jasnell@gmail.com>