Function download_extension 

fn download_extension(
    binary: &str,
    platform_key: &str,
    version: &str,
    metadata: &ReleaseBinary,
    public_key: &PublicKey,
    download_dir: &Path,
    dry_run: bool,
) -> Result<Option<PathBuf>, InstallerError>

Downloads an extension binary, verifies its checksum and signature, and returns the path to the verified file in download_dir.