autocomplete: Introduce topic input autocomplete

lib/api/route/channels.dart

@@ -0,0 +1,40 @@
+import 'package:json_annotation/json_annotation.dart';
+
+import '../core.dart';
+part 'channels.g.dart';
+
+/// https://zulip.com/api/get-stream-topics
+Future<GetStreamTopicsResult> getStreamTopics(ApiConnection connection, {
+  required int streamId,
+}) {
+  return connection.get('getStreamTopics', GetStreamTopicsResult.fromJson, 'users/me/$streamId/topics', {});
+}
+
+@JsonSerializable(fieldRename: FieldRename.snake)
+class GetStreamTopicsResult {
+  final List<GetStreamTopicsEntry> topics;
+
+  GetStreamTopicsResult({
+    required this.topics,
+  });
+
+  factory GetStreamTopicsResult.fromJson(Map<String, dynamic> json) =>
+    _$GetStreamTopicsResultFromJson(json);
+
+  Map<String, dynamic> toJson() => _$GetStreamTopicsResultToJson(this);
+}
+
+@JsonSerializable(fieldRename: FieldRename.snake)
+class GetStreamTopicsEntry {
+  final int maxId;
+  final String name;
+
+  GetStreamTopicsEntry({
+    required this.maxId,
+    required this.name,
+  });
+
+  factory GetStreamTopicsEntry.fromJson(Map<String, dynamic> json) => _$GetStreamTopicsEntryFromJson(json);
+
+  Map<String, dynamic> toJson() => _$GetStreamTopicsEntryToJson(this);
+}