net: doc: Add information of DNS service discovery

:zephyr_file:`subsys/net/lib/dns/Kconfig`. The DNS resolver API can be found at

:zephyr_file:`include/zephyr/net/dns_resolve.h`.

DNS-based service discovery queries described in

`IETF RFC6763 <https://datatracker.ietf.org/doc/html/rfc6763>`_

can be done by :c:func:`dns_resolve_service` API.

The returned service descriptions are passed to user supplied callback

and the API sets the address family to :c:macro:`AF_LOCAL` to indicate that

the value is not an IPv4 or IPv6 address but a service description.

Example:

.. code-block:: c

   #include <zephyr/net/dns_resolve.h>

   #define MAX_STR_LEN CONFIG_DNS_RESOLVER_MAX_NAME_LEN

   static void dns_result_cb(enum dns_resolve_status status,

                             struct dns_addrinfo *info,

                             void *user_data)

   {

        if (status == DNS_EAI_CANCELED) {

                /* dns: Timeout while resolving name */

                return;

	}

        if (status == DNS_EAI_INPROGRESS && info) {

                char str[MAX_STR_LEN + 1];

                if (info->ai_family == AF_INET) {

                        net_addr_ntop(AF_INET,

                                      &net_sin(&info->ai_addr)->sin_addr,

                                      str, NET_IPV4_ADDR_LEN);

                } else if (info->ai_family == AF_INET6) {

                        net_addr_ntop(AF_INET6,

                                      &net_sin6(&info->ai_addr)->sin6_addr,

                                      str, NET_IPV6_ADDR_LEN);

                } else if (info->ai_family == AF_LOCAL) {

                        /* service discovery */

                        memset(str, 0, MAX_STR_LEN);

                        memcpy(str, info->ai_canonname,

                               MIN(info->ai_addrlen, MAX_STR_LEN));

                } else {

                        strncpy(str, "Invalid proto family", MAX_STR_LEN + 1);

                }

                str[MAX_STR_LEN] = '\0';

                printk("dns: %s\n", str);

                return;

        }

        if (status == DNS_EAI_ALLDONE) {

                printk("dns: All results received\n");

                return;

        }

        if (status == DNS_EAI_FAIL) {

                printk("dns: No such name found.\n");

                return;

        }

        printk("dns: Unhandled status %d received (errno %d)\n", status, errno);

   }

   #define DNS_TIMEOUT (MSEC_PER_SEC * 5) /* in ms */

   static void discover_service(void)

   {

        int ret = dns_resolve_service(dns_resolve_get_default(),

                                      "_http._tcp.dns-sd.org",

                                      NULL, dns_result_cb,

                                      NULL, DNS_TIMEOUT);

        ...

   }

The above query would return output like this:

.. code-block: console

    Query for '_http._tcp.dns-sd.org' sent.

    dns: . * cnn, world news._http._tcp.dns-sd.org

    dns: .source de télévision, département de langues._http._tcp.dns-sd.org

    dns: . * multicast dns._http._tcp.dns-sd.org

    dns: . * amazon.com, on-line shopping._http._tcp.dns-sd.org

    dns: . * google, searching the web._http._tcp.dns-sd.org

    dns: . * ebay, online auctions._http._tcp.dns-sd.org

    dns: . * apple, makers of the ipod._http._tcp.dns-sd.org

    dns: . * yahoo, maps, weather, and stock quotes._http._tcp.dns-sd.org

    dns: .about bonjour in web browsers._http._tcp.dns-sd.org

    dns: .π._http._tcp.•bullets•.dns-sd.org

    dns: . * dns service discovery._http._tcp.dns-sd.org

    dns: . * wired, technology, culture, business, politics._http._tcp.dns-sd.org

    dns: . * slashdot, news for nerds, stuff that matters._http._tcp.dns-sd.org

    dns: . * bbc, world news._http._tcp.dns-sd.org

    dns: .stuart’s printer._http._tcp.dns-sd.org

    dns: . * zeroconf._http._tcp.dns-sd.org

    dns: All results received

As the service discovery query could return long strings and the packet size could

be large, you might need to adjust following Kconfig options:

- :kconfig:option:`CONFIG_DNS_RESOLVER_MAX_ANSWER_SIZE`. This tells the maximum size of the

  answer, typical value for this option could be 1024. The default size for this option is

  512 bytes.

- :kconfig:option:`CONFIG_DNS_RESOLVER_MAX_NAME_LEN`. This tells the maximum length of the

  returned name. The value depends on your expected data size, typical value might be 128 bytes.

Sample usage

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