esp_openthread_get_dnsserver_addr() function

- ESP_OK on sussess - ESP_ERR_INVALID_ARG if dnsserver_addr is NULL or obtained DNS server address is invalid - ESP_ERR_ESP_NETIF_IF_NOT_READY if openthread network interface is not initialized - ESP_ERR_ESP_NETIF_INVALID_PARAMS if failed to call esp_netif APIs getting dns info