Grub needs absolute paths to initrd and vmlinuz if we do not have
`/boot` in a boot partition, which we do not in bootc.

Add param `boot_subdir` which acts like a subdirectory in the boot
directory in case the boot partition is mounted in another directory.

Signed-off-by: Pragyan Poudyal <pragyanpoudyal41999@gmail.com>

It turns out that the information contained in splitstreams to assist
with garbage collection (ie: the list of things that we mustn't discard)
is exactly the required information for downloading (ie: the list of
things that we must acquire).

Use this fact to add support for fetching repository content from HTTP
servers.  We only download the objects that are actually required, so
incremental pulls are very fast.

This works with just about any HTTP server, so you can do something like

  python -m http.server -d ~/.var/lib/composefs

and download from that.  With a fast enough web server on localhost,
pulling a complete image into an empty repository takes about as long as
pulling an `oci:` directory via skopeo with `cfsctl oci pull`.

In practice, this is intended to be used with a webserver which supports
static compression and pre-compressed objects stored on the server.  In
particular, zstd support is enabled in the `reqwest` crate for this
reason, and it's working with something like:

  find repo/objects/ -type f -name '*[0-9a-f]' -exec zstd -19 -v '{}' +
  static-web-server -p 8888 --compression-static -d repo

There's also an included s3-uploader.py in the examples/ directory which
will upload a repository to an S3 bucket, with zstd compression.

Signed-off-by: Allison Karlitskaya <allison.karlitskaya@redhat.com>

Change the Repository::mount() API to return the mounted filesystem as
an fd rather than taking the mountpoint as an argument.  Create a new
mount_at() API to replace the old one, replacing the canicalize() and
mount_at() calls that used to be in mount_composefs_at(), which we
remove.

Update the various users.

Making this change lets us simplify the logic in composefs-setup-root:
it no longer has to manually open the image in order to perform the
fsmount operation: it can use the new API on the repository.

This allows us to make Repository::open_image() private, so do that too.

Co-authored-by: Sanne Raymaekers <sanne.raymaekers@gmail.com>
Signed-off-by: Allison Karlitskaya <allison.karlitskaya@redhat.com>

Allows cfsctl operations with fs-verity disabled.

Signed-off-by: Sanne Raymaekers <sanne.raymaekers@gmail.com>

Supports writing `composefs=?<root-id>`.

Signed-off-by: Sanne Raymaekers <sanne.raymaekers@gmail.com>

Signed-off-by: Pragyan Poudyal <pragyanpoudyal41999@gmail.com>

…28c985b06'

git-subtree-dir: crates/lib/cfsctl
git-subtree-mainline: 07faf7d
git-subtree-split: b46b1f7