IPv6 preview for ESP32, also working for ESP8266

[s-hadinger]

Description:

IPv6 preview for ESP32. It's still experimental and for testing only:

• support for IPv6 on ESP8266 and ESP32
• automatically generate link-local IPv6 address fe80::...
• DHCPv6 support for assigning global IPv6
• Berry tasmota.wifi() extended to support IPv6
• Status 5 extended to report IPv6

Exemple:

00:00:00.529 WIF: Connecting to AP1 Tasmota Channel 1 BSSId xx:xx:xx:xx:xx:xx in mode 11n as tasmota-xxxxxx-xxxx...
00:00:02.761 WIF: Connected
00:00:02.763 WIF: IPv6 Link-Local fe80::xxxx:xxxx:xxxx:xxxx
00:00:03.015 HTP: Web server active on tasmota-67B1E9-4585 with IP address 192.168.x.xxx

Status 5

20:45:38.946 RSL: STATUS5 = {"StatusNET":{"Hostname":"tasmota-xxxxxx-xxxx","IPAddress":"192.168.x.xxx","Gateway":"192.168.x.x","Subnetmask":"255.255.255.0","DNSServer1":"192.168.x.x","DNSServer2":"0.0.0.0","Mac":"xx:xx:xx:xx:xx:xx","IP6Global":"2a01:cb00:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx","IP6Local":"fe80::xxxx:xxxx:xxxx:xxxx","Webserver":2,"HTTP_API":1,"WifiConfig":2,"WifiPower":17.0}}

How-to make it work:

• ESP8266: enable -DPIO_FRAMEWORK_ARDUINO_LWIP2_IPV6_HIGHER_BANDWIDTH build option instead of -DPIO_FRAMEWORK_ARDUINO_LWIP2_HIGHER_BANDWIDTH_LOW_FLASH
• ESP32: requires a specific build for IPv6 support, can be tested with platform_packages = framework-arduinoespressif32 @ https://github.com/Jason2866/esp32-arduino-lib-builder/releases/download/1051/framework-arduinoespressif32-release_v4.4-e7f0f8b3f4.zip

What's not working or not tested:

• SLAAC is supposed to work but not tested
• WebUI does not listen on IPv6 address (yet)
• MQTT is untested with IPv6 MQTTHost
• Wifi only, Ethernet not tested

Related issue (if applicable): fixes #2521

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.4.9
• The code change is tested and works with Tasmota core ESP32 V.2.0.5
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass