load abstract method
Loads a snapshot from storage.
Parameters:
key: Optional key/identifier for the snapshot. If not provided, the default/latest snapshot will be loaded.
Returns:
A Future that completes with the loaded Snapshot, or null if
no snapshot exists for the given key.
Throws:
- FormatException if the stored data is invalid
- Storage-specific exceptions (e.g., IOException for file storage)
Implementation
Future<Snapshot?> load([String? key]);