Add suppport for PuzzleFS images in the oci template

Add suppport for PuzzleFS images in the oci template

PuzzleFS images (media type 'application/vnd.puzzlefs.image.rootfs.v1')
can be mounted in a similar way to squashfs images, we just have to
detect the type and reuse the existing code for providing a mount
helper. PuzzleFS is a next-generation container filesystem [1] with
several benefits, such as  reduced duplication, reproducible image
builds, direct mounting support and memory safety guarantees.

Since PuzzleFS currently doesn't provide an image config, also add
support for empty image configs, they are supported by the OCI spec [2].

The MOUNT_HELPER is now passed a `--persist <upperdir>` flag, so it
knows that it needs to create an overlay. This is needed because LXC
expects a writable rootfs and both atomfs and puzzlefs are read-only
filesystems.

Example:
```
$ sudo env PATH=$PATH build/src/lxc/tools/lxc-create --name mycontainer -t \
oci -- --url oci:/$HOME/.local/share/puzzlefs/pfs_ubuntu:eg --no-cache

$ sudo build/src/lxc/tools/lxc-start --name mycontainer --foreground /bin/bash
```

--no-cache is needed for puzzlefs until [3] is solved

[1] https://github.com/project-machine/puzzlefs
[2] https://github.com/opencontainers/image-spec/blob/main/manifest.md#image-manifest
[3] https://github.com/project-machine/puzzlefs/issues/131

Signed-off-by: Ariel Miculas-Trif <amiculas@cisco.com>