Function fetch_and_install_bundle 

pub fn fetch_and_install_bundle(
    url: &str,
    zip_sha256: &str,
    opts: &FetchOpts,
) -> Result<(PathBuf, String)>

Download a jan bundle zip, verify the zip SHA256, verify manifest members, unpack under ~/.cache/jan/trees/<sha256>/, and return (tree_dir, root_yaml).