Add TestSuite Model with Serialization and Deserialization Support

This commit introduces a new class, TestSuite, designed to encapsulate a collection of Task objects under a common timestamp. This will help in grouping tasks that belong to a particular test suite.

Key Features:
- Add a TestSuite class with fields for `timestamp` and a list of `tests` (Task objects).
- Implement `toJson` method for serializing TestSuite objects to JSON-compatible format.
- Implement `fromJson` factory method for deserializing JSON data back into a TestSuite object.

By providing serialization and deserialization support directly in the model, we facilitate easier storage and data exchange for test suites.

frontend/lib/models/test_suite.dart

@@ -0,0 +1,25 @@
+import 'package:auto_gpt_flutter_client/models/task.dart';
+
+class TestSuite {
+  final String timestamp;
+  final List<Task> tests;
+
+  TestSuite({required this.timestamp, required this.tests});
+
+  // Serialization: Convert the object into a Map
+  Map<String, dynamic> toJson() {
+    return {
+      'timestamp': timestamp,
+      'tests': tests.map((task) => task.toJson()).toList(),
+    };
+  }
+
+// Deserialization: Create an object from a Map
+  factory TestSuite.fromJson(Map<String, dynamic> json) {
+    return TestSuite(
+      timestamp: json['timestamp'],
+      tests: List<Task>.from(json['tests'].map(
+          (taskJson) => Task.fromMap(Map<String, dynamic>.from(taskJson)))),
+    );
+  }
+}