Adding "captive portal" functionality to `EspBootstrap::run()`

[jostsalathe]

I thought that the functionality that can often be seen on public WiFi networks where you are automatically directed to a login page when connecting to that network would be quite useful for the EspBootstrap use case.

During searching for solutions I found out that this is apparently called a "captive portal" and that it can be implemented by having a DNS server on the Access Point like it is done here: https://github.com/125K/ESP8266-Captive-Portal/blob/master/WiFi_Captive_Portal.ino

I just tried adding the DNS server to EspBootstrapDict::doRun() like so:

#include <Arduino.h>
#include <DNSServer.h> //============================================= ADDED THIS
#include <EspBootstrapBase.h>
#include <Dictionary.h>

/*[...]*/

int8_t EspBootstrapDict::doRun() {

  String ssid(SSID_PREFIX);
  const IPAddress   APIP   (10, 1, 1, 1);
  const IPAddress   APMASK (255, 255, 255, 0);

  WiFi.disconnect();
  WiFi.mode(WIFI_AP);

  ssid += WiFi.macAddress();
  ssid.replace(":", "");
  ssid.toLowerCase();
//#if defined( ARDUINO_ARCH_ESP8266 )
//  ssid += String(ESP.getChipId(), HEX);
//#endif
#if defined( ARDUINO_ARCH_ESP32 )
//  ssid += String((uint32_t)( ESP.getEfuseMac() & 0xFFFFFFFFL ), HEX);
  WiFi.softAP( ssid.c_str());
  delay(50);
#endif

  WiFi.softAPConfig(APIP, APIP, APMASK);
  delay(50);
  WiFi.softAP( ssid.c_str());
  yield();

  DNSServer dnsServer; //============================================= ADDED THIS
  iServer = new WebServer(80);
  if (iServer == NULL) return BOOTSTRAP_ERR;

  dnsServer.start(53, "*", APIP); // DNS spoofing (Only for HTTP) //== ADDED THIS

  iServer->on("/submit.html", __espbootstrap_handlesubmit);
  iServer->onNotFound(__espbootstrap_handleroot);

  iAllDone = false;
  iServer->begin();

  uint32_t timeNow = millis();
  while (!iAllDone && !iCancelAP) {
    dnsServer.processNextRequest(); //================================ ADDED THIS
    iServer->handleClient();
    if ( millis() - timeNow > iTimeout ) {
        iServer->stop();
        iServer->close();
        delete iServer;
        iServer = NULL;
        return BOOTSTRAP_TIMEOUT;
    }
    delay(10);
//    yield();
  }

  iServer->stop();
  iServer->close();
  delete iServer;
  iServer = NULL;
  return (iCancelAP ? BOOTSTRAP_CANCEL: BOOTSTRAP_OK);
}

It worked just fine when connecting with my smartphone! My PC probably didn't do it because it already has a wired connection with another DNS Server answering.

I don't know enough about the structure of the EspBootstrap library but I would be toatally stoked if this feature would find its way into it since I think that it adds a really nice step to streamlining the setup process for devices.