Expose function for creating a new session out of an exported snap
Reads the salt used when exporting from the `<exportedSnap>/salt` file.
Reads the salt used when exporting from the `<exportedSnap>/salt` file.
Importing a snapshot requires the same salt that was used when exporting it. We copy the `<session>/metadata` file into the exported snapshot as `<exportedSnap>/salt` so that we can recover it.