Calculates the signature on all the data passed through using either sign.update() or sign.write().
If privateKey is not a KeyObject, this function behaves as if privateKey had been passed to createPrivateKey. If it is an object, the following additional properties can be passed:
If outputEncoding is provided a string is returned; otherwise a Buffer is returned.
The Sign object can not be again used after sign.sign() method has been called. Multiple calls to sign.sign() will result in an error being thrown.