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
Signed-off-by: Aditya Ramani <a_ramani@apple.com>