Function pull_module 

pub fn pull_module(
    artifact_ref: &str,
    output_dir: &Path,
    require_signature: bool,
    printer: Option<&Printer>,
) -> Result<(), OciError>

Pull a module from an OCI registry and extract it to output_dir.

If require_signature is true, checks for a cosign signature tag (<tag>.sig or sha256-<hash>.sig) and returns an error if not found.