feat: add DNS-SD CoAP example

example/coap_dns_sd_discovery.dart

@@ -0,0 +1,48 @@
+// Copyright 2023 The NAMIB Project Developers. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+//
+// SPDX-License-Identifier: BSD-3-Clause
+
+// ignore_for_file: avoid_print
+
+import 'package:dart_wot/dart_wot.dart';
+
+const propertyName = 'string';
+
+extension PrintExtension on InteractionOutput {
+  Future<void> printValue() async {
+    print(await value());
+  }
+}
+
+void handleThingDescription(ThingDescription thingDescription) =>
+    print('Discovered TD with title "${thingDescription.title}".');
+
+Future<void> main(List<String> args) async {
+  final servient = Servient()..addClientFactory(CoapClientFactory());
+
+  final wot = await servient.start();
+  final uri = Uri.parse('_wot._udp.local');
+
+  // Example using for-await-loop
+  try {
+    await for (final thingDescription
+        in wot.discover(uri, method: DiscoveryMethod.dnsServiceDiscovery)) {
+      handleThingDescription(thingDescription);
+    }
+    print('Discovery with "await for" has finished.');
+  } on Exception catch (error) {
+    print(error);
+  }
+
+  // Example using the .listen() method, allowing for error handling
+  //
+  // Notice how the "onDone" callback is called before the result is passed
+  // to the handleThingDescription function.
+  wot.discover(uri, method: DiscoveryMethod.dnsServiceDiscovery).listen(
+        handleThingDescription,
+        onError: (error) => print('Encountered an error: $error'),
+        onDone: () => print('Discovery with "listen" has finished.'),
+      );
+}