code: static byte[] payloadAndAuthTag(OmemoElement element, byte[] authTag) {
          if (!element.isMessageElement()) {
              throw new IllegalArgumentException("OmemoElement has no payload.");
          }
  
          byte[] payload = new byte[element.getPayload().length + authTag.length];
          System.arraycopy(element.getPayload(), 0, payload, 0, element.getPayload().length);
          System.arraycopy(authTag, 0, payload, element.getPayload().length, authTag.length);
          return payload;
      }
docs: Return the concatenation of the payload of the OmemoElement and the given auth tag.
  
  @param element omemoElement (message element)
  @param authTag authTag
  @return payload + authTag