implemented full interactive poll support in Talk, including creation, voting, and closing functionality

lib/api/marianumcloud/talk/create_poll/create_poll.dart

@@ -0,0 +1,31 @@
+import 'dart:convert';
+
+import 'package:http/http.dart' as http;
+
+import '../../../api_params.dart';
+import '../talk_api.dart';
+import 'create_poll_params.dart';
+
+/// Erstellt eine Umfrage; der Server postet die Poll-Nachricht selbst in den
+/// Chat, danach genügt ein Chat-Refresh. Nur in Gruppen-Chats erlaubt.
+class CreatePoll extends TalkApi {
+  CreatePoll({required String token, required CreatePollParams params})
+    : super(
+        'v1/poll/$token',
+        params,
+        headers: {'Content-Type': 'application/json'},
+      );
+
+  @override
+  Null assemble(String raw) => null;
+
+  @override
+  Future<http.Response>? request(
+    Uri uri,
+    ApiParams? body,
+    Map<String, String>? headers,
+  ) {
+    if (body is! CreatePollParams) return null;
+    return http.post(uri, headers: headers, body: jsonEncode(body.toJson()));
+  }
+}