Define a UnpackStrategy function type in the SnapshotStore to give more control over how an image is unpacked.

Previously, we were creating a 512 GB sparse block file for the initial file system of a container, which is overkill.

With this change, the vminit image is unpacked to a smaller block file, while container images are unpacked to the 512 GB block

Follows the same pattern as https://github.com/apple/container/blob/main/Sources/Helpers/RuntimeLinux/RuntimeLinuxHelper.swift#L71