Function handle_verify 

pub async fn handle_verify(
    file: &Path,
    signature: Option<PathBuf>,
    identity_bundle: Option<PathBuf>,
    witness_receipts: Option<PathBuf>,
    witness_keys: &[String],
    witness_threshold: usize,
) -> Result<()>

Execute the artifact verify command.

Exit codes: 0=valid, 1=invalid, 2=error.