Added non-blocking connect method

[KrazyKerbalnaut]

This is not completely non-blocking. Please review the delay of 100ms after setting passphrase or key of the normal "begin()" method. To resolution is a little un-optimized at the moment but I needed to to work for now:

WiFi.cpp:

int WiFiClass::setCredNB(const char* ssid, const char *passphrase)
{
  uint8_t status = WL_CONNECT_FAILED;
  startConnection = millis();
  delayStartConnection = millis();

  if (WiFiDrv::wifiSetPassphrase(ssid, strlen(ssid), passphrase, strlen(passphrase))!= WL_FAILURE) {
    status = WiFiDrv::getConnectionStatus();
  }

  return status;
}

int WiFiClass::beginNB()
{
  uint8_t status = WL_CONNECT_FAILED;

  if ((millis() - startConnection) < _timeout) {
    if ((millis() - delayStartConnection) > WL_DELAY_START_CONNECTION) {
      status = WiFiDrv::getConnectionStatus();
      delayStartConnection = millis();
    }
  }

  return status;
}

WiFi.h, add to public section:

unsigned long startConnection;
unsigned long delayStartConnection;

int setCredNB(const char* ssid, const char* passphrase);
int beginNB();

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