Add API to create a Snapshot without going through creating a VM partition

Ok, I think the way to go would be to add a new builder terminator `snapshot_from_file` and `snapshot_from_bytes`, and use those to benchmark all the setup that we were measuring for the uninitialized sandbox here (maybe except for the memory allocation and memcpy, but that's not a big loss).

I'll create an issue to add this and reintroduce the new benchmarks to compensate what we loose here.

_Originally posted by @jprendes in https://github.com/hyperlight-dev/hyperlight/pull/1746#discussion_r3829469340_

Start with the builder API and the uninitialized-sandbox benchmarks discussed in PR #1746, then trace the existing snapshot creation path. The issue names snapshot_from_file and snapshot_from_bytes as the entry points; done means snapshots can be created through them without creating a VM partition and the relevant setup benchmarks are restored.

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