snippets: Add Wi-Fi IPv6 snippet

Introduce a snippet for configuring IPv6 over Wi-Fi support in
networking samples.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>

snippets/wifi-ipv6/README.rst

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+.. _snippet-wifi-ipv6:
+
+Wi-Fi IPv6 Snippet (wifi-ipv6)
+##############################
+
+.. code-block:: console
+
+   west build -S wifi-ipv6 [...]
+
+Overview
+********
+
+This snippet enables IPv6 Wi-Fi support in supported networking samples.
+The sample execution is postponed until Wi-Fi connectivity is established.
+
+Use Wi-Fi shell to connect to the Wi-Fi network:
+
+.. code-block:: console
+
+   wifi connect -s <SSID> -k <key_management> -p <passphrase>
+
+Requirements
+************
+
+Hardware support for:
+
+- :kconfig:option:`CONFIG_WIFI`
+- :kconfig:option:`CONFIG_WIFI_USE_NATIVE_NETWORKING`

snippets/wifi-ipv6/snippet.yml

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+name: wifi-ipv6
+append:
+  EXTRA_CONF_FILE: wifi-ipv6.conf

snippets/wifi-ipv6/wifi-ipv6.conf

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+# Wi-Fi
+CONFIG_WIFI=y
+CONFIG_NET_L2_ETHERNET=y
+CONFIG_WIFI_NM_WPA_SUPPLICANT=y
+
+# Make sure there is enough resources for supplicant and most of the samples
+CONFIG_MAIN_STACK_SIZE=2048
+CONFIG_ZVFS_OPEN_MAX=24
+CONFIG_NET_MAX_CONN=10
+CONFIG_ZVFS_POLL_MAX=10
+
+# IPv6 only for now
+CONFIG_NET_IPV6=y
+CONFIG_NET_CONFIG_NEED_IPV4=n
+CONFIG_NET_IPV4=n
+
+# DHCPv6
+CONFIG_NET_DHCPV6=y
+CONFIG_NET_CONFIG_MY_IPV6_ADDR=""
+CONFIG_NET_CONFIG_INIT_TIMEOUT=0
+
+# DNS
+CONFIG_DNS_RESOLVER=y
+
+# Connection manager
+CONFIG_NET_CONNECTION_MANAGER=y
+
+# Wi-Fi shell
+CONFIG_NET_SHELL=y
+CONFIG_NET_L2_WIFI_SHELL=y
+CONFIG_SHELL_STACK_SIZE=5200