feat: implement requestThingDescription method

lib/src/binding_coap/coap_client.dart

@@ -538,4 +538,12 @@ final class CoapClient implements ProtocolClient {
       yield content;
     }
   }
+
+  @override
+  Future<Content> requestThingDescription(Uri url) async => _sendRequest(
+        url,
+        coap.RequestMethod.get,
+        form: null,
+        accept: coap.CoapMediaType.applicationTdJson,
+      );
 }

lib/src/binding_http/http_client.dart

@@ -332,4 +332,18 @@ final class HttpClient implements ProtocolClient {
 
     yield encodedLinks;
   }
+
+  @override
+  Future<Content> requestThingDescription(Uri url) async {
+    final request = Request(HttpRequestMethod.get.methodName, url);
+    const tdContentType = 'application/td+json';
+    request.headers['Accept'] = tdContentType;
+
+    final response = await _client.send(request);
+
+    return Content(
+      response.headers['Content-Type'] ?? tdContentType,
+      response.stream,
+    );
+  }
 }

lib/src/binding_mqtt/mqtt_client.dart

@@ -251,4 +251,10 @@ final class MqttClient implements ProtocolClient {
     // TODO: implement discoverWithCoreLinkFormat
     throw UnimplementedError();
   }
+
+  @override
+  Future<Content> requestThingDescription(Uri url) {
+    // TODO: implement requestThingDescription
+    throw UnimplementedError();
+  }
 }

lib/src/core/protocol_interfaces/protocol_client.dart

@@ -62,4 +62,7 @@ abstract interface class ProtocolClient {
     void Function(Exception error)? error,
     required void Function() complete,
   });
+
+  /// Requests a Thing Description as [Content] from a [url].
+  Future<Content> requestThingDescription(Uri url);
 }

lib/src/core/servient.dart

@@ -15,6 +15,7 @@ import 'exposed_thing.dart';
 import 'protocol_interfaces/protocol_client.dart';
 import 'protocol_interfaces/protocol_client_factory.dart';
 import 'protocol_interfaces/protocol_server.dart';
+import 'thing_discovery.dart';
 import 'wot.dart';
 
 /// Exception that is thrown by a [Servient].
@@ -225,4 +226,20 @@ class Servient {
 
     return clientFactory.createClient();
   }
+
+  /// Requests a [ThingDescription] from a [url].
+  Future<ThingDescription> requestThingDescription(Uri url) async {
+    final client = clientFor(url.scheme);
+    final content = await client.requestThingDescription(url);
+
+    final value = await contentSerdes.contentToValue(content, null);
+
+    if (value is! Map<String, dynamic>) {
+      throw DiscoveryException(
+        'Could not parse Thing Description obtained from $url',
+      );
+    }
+
+    return ThingDescription.fromJson(value);
+  }
 }

lib/src/core/wot.dart

@@ -90,4 +90,9 @@ class WoT implements scripting_api.WoT {
   }) {
     return ThingDiscovery(url, thingFilter, _servient, method: method);
   }
+
+  @override
+  Future<ThingDescription> requestThingDescription(Uri url) {
+    return _servient.requestThingDescription(url);
+  }
 }

lib/src/scripting_api/wot.dart

@@ -31,6 +31,9 @@ abstract interface class WoT {
   /// based on the underlying impementation.
   Future<ExposedThing> produce(ExposedThingInit exposedThingInit);
 
+  /// Requests a [ThingDescription] from the given [url].
+  Future<ThingDescription> requestThingDescription(Uri url);
+
   /// Discovers [ThingDescription]s from a given [url] using the specified
   /// [method].
   ///