common: ipv6_only package for the IPv6-only IoT network

- common/wifi.yaml: the network gets id wifi_iot, so devices can
  override it with '!extend wifi_iot'
- common/ipv6_only.yaml: add-on package next to any common package;
  switches to the IPv6-only SSID, wifi ipv6_only (SLAAC, no DHCPv4,
  esphome/esphome#19750 as external component until merged) and
  min_ipv6_addr_count: 1
- mcu-lvgl-trainer, mcu-nspanel-dev, mcu-rfid-reader, mcu-stackchan,
  mcu-tft moved to the IPv6-only network

common/wifi.yaml

@@ -2,7 +2,9 @@
 wifi:
   # reboot_timeout: 0s
   networks:
-    - ssid: !secret wifi_ssid
+    # id lets devices override the network with '!extend wifi_iot'
+    - id: wifi_iot
+      ssid: !secret wifi_ssid
       password: !secret wifi_password
   enable_btm: true
   enable_rrm: true