Add LeaderboardService with submitReport Method

This commit adds a new `LeaderboardService` class featuring a `submitReport` method. This method allows for the submission of `BenchmarkRun` objects to the leaderboard via a POST request to the `/api/reports` endpoint. The new service uses the `ApiType.leaderboard` enum value.

frontend/lib/services/leaderboard_service.dart

@@ -0,0 +1,24 @@
+import 'package:auto_gpt_flutter_client/models/benchmark/api_type.dart';
+import 'package:auto_gpt_flutter_client/models/benchmark/benchmark_run.dart';
+import 'package:auto_gpt_flutter_client/utils/rest_api_utility.dart';
+
+class LeaderboardService {
+  final RestApiUtility api;
+
+  LeaderboardService(this.api);
+
+  /// Submits a benchmark report to the leaderboard.
+  ///
+  /// [benchmarkRun] is a BenchmarkRun object representing the data of a completed benchmark.
+  Future<Map<String, dynamic>> submitReport(BenchmarkRun benchmarkRun) async {
+    try {
+      return await api.post(
+        'api/reports',
+        benchmarkRun.toJson(),
+        apiType: ApiType.leaderboard,
+      );
+    } catch (e) {
+      throw Exception('Failed to submit the report to the leaderboard: $e');
+    }
+  }
+}