LEAmDNS v2 (optional) from @LaborEtArs

[d-a-v]

This is a work in progress provided by @LaborEtArs with some minor ajustments.

This PR contains two more mDNS implementations providing:

• IPv6 support
• support for multiple and independent interfaces
  (an example is provided in examples/LEAmDNS/TwoInterfaces)

To help description, here's the updated ESP8266mDNS.h:

enum class MDNSApiVersion { Legacy, LEA, LEAv2Compat, LEAv2 };

#include "ESP8266mDNS_Legacy.h" // Legacy
#include "LEAmDNS.h"            // LEA
#include "LEAmDNS2_Legacy.h"    // LEAv2Compat - replacement for LEA using v2 API
#include "LEAmDNS2Host.h"       // LEAv2       - API updated

// clsMDNSHost replaces MDNSResponder in LEAv2
using clsMDNSHost = esp8266::experimental::clsLEAMDNSHost;

#if !defined(NO_GLOBAL_INSTANCES) && !defined(NO_GLOBAL_MDNS)
// Maps the implementation to use to the global namespace type
//using MDNSResponder = Legacy_MDNSResponder::MDNSResponder;                // Legacy
using MDNSResponder = esp8266::MDNSImplementation::MDNSResponder;           // LEA
//using MDNSResponder = esp8266::MDNSImplementation::clsLEAMDNSHost_Legacy; // LEAv2Compat
//using MDNSResponder = clsMDNSHost;                                        // LEAv2

extern MDNSResponder MDNS;
#endif

New implementations are:

• clsMDNSHost (= esp8266::experimental::clsLEAMDNSHost)
  new but partially compatible API
• esp8266::MDNSImplementation::clsLEAMDNSHost_Legacy
  Wrapper on top of new API to provide current API (to be tested)

The new partially compatible API (MDNSResponder = clsMDNSHost) works with most of the simple examples (including OTA - @LaborEtArs I added clsLEAMDNSHost::enableArduino()).
Also mDNS_clock example is provided for both versions.

In its current state, Legacy LEAmDNS API is kept as default.

What has been verified:

• Former LEAmDNS is enabled by default
• New API works on both interfaces with the TwoInterfaces example
• Simple sketches using mDNS work with the new API
  (I can't see an IPv6 address but there's an IPv6 answer):

+ wlp2s0 IPv6 arduino                                       _arduino._tcp        local
+ wlp2s0 IPv4 arduino                                       _arduino._tcp        local
= wlp2s0 IPv6 arduino                                       _arduino._tcp        local
   hostname = [OTA-LEDS.local]
   address = [10.0.1.112]
   port = [8266]
   txt = ["auth_upload=no" "board="ESP8266_WEMOS_D1MINIPRO"" "ssh_upload=no" "tcp_check=no"]
= wlp2s0 IPv4 arduino                                       _arduino._tcp        local
   hostname = [OTA-LEDS.local]
   address = [10.0.1.112]
   port = [8266]
   txt = ["auth_upload=no" "board="ESP8266_WEMOS_D1MINIPRO"" "ssh_upload=no" "tcp_check=no"]

What has to be doubled checked:

• using MDNSResponder = esp8266::MDNSImplementation::clsLEAMDNSHost_Legacy; // LEAv2Compat

Current exploration:

• A single instance shared on all interfaces as an option

[d-a-v]

Closing in favor of #7335