Support embedding user-provided data in persisted snapshots

[ludfjig]

Hyperlight-wasm and Hyperlight-js could use this to prevent accidental misuse for loading hyperlight snapshots. For example (proposed api) LoadedJsSnapshot::load(...) API should reject arbitrary hyperlight snapshots. It should validate that a snapshot represents a loaded JS sandbox saved through (proposed api) LoadedJSSandbox::snapshot(&mut self).save() API. But there's currently no way to do this.

[ludfjig]

Maybe extend Snapshot::save(path, tag) and Snapshot::load(path, reference) APIs, and add variants that accept and return user-provided data:

pub fn save_with_data(
    &self,
    path: impl AsRef<Path>,
    tag: &OciTag,
    data: &[u8],
) -> Result<OciDigest>;

pub fn load_with_data(
    path: impl AsRef<Path>,
    reference: impl Into<OciReference>,
) -> Result<(Self, Vec<u8>)>;