Add transformer for container roots

This change renames the root transformer to indicate that it operates on host paths and adds a container root transformer for explicitly transforming container roots.

This means that running:

nvidia-ctk transform root --from=/foo/bar --to=/ --relative-to=container

will update the container roots in the CDI spec.

This is a breaking change for consumers of the NewRootTransformer API.

Signed-off-by: Evan Lezar elezar@nvidia.com