add blocking API

[bertmelis]

@s0170071

Add blocking API

// signatures
VitoWiFi::OptolinkResult  readBlocking(Datapoint datapoint);
VitoWiFi::OptolinkResult  writeBlocking(Datapoint datapoint, const uint8_t* data, uint8_t length);
VitoWiFi::OptolinkResult  writeBlocking(Datapoint datapoint, const uint8_t* data);

// result can be evaluated as below
if (result == VitoWiFi::Result::SUCCESS) {
  uint8_t* data = vitoWiFi.getResponseData();
  std::size_t len = vitoWiFi.getResponseLength();
  VitoWiFi::PacketVS2 vitoWiFi.getResponse();
} else {
  const char* errorString = VitoWiFi::errorToString(result);
}

I have not started on this yet so very much open for discussion.

Conversion of the data to string will be implemented separately (see https://github.com/bertmelis/VitoWiFi/tree/convert-to-string)