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

mcu-nspanel-dev.yaml

@@ -59,6 +59,7 @@ external_components:
 
 packages:
   common: !include common/common.yaml
+  ipv6_only: !include common/ipv6_only.yaml
   # common: !include common/common_dev.yaml
 
 # web_server: