Introduce TaskService Class for Task Operations

This commit adds a new TaskService class to handle all task-related operations, including task creation, listing, and artifacts management.

- Implemented methods for creating a new task (createTask).
- Added functionality to retrieve details for a specific task (getTaskDetails).
- Enabled listing all tasks with optional pagination (listAllTasks).
- Enabled listing all artifacts for a specific task with optional pagination (listTaskArtifacts).

By encapsulating these operations within the TaskService class, this commit provides a clean and centralized way to interact with the backend for task functionalities, making the application more maintainable and easier to extend.

lib/services/task_service.dart

@@ -0,0 +1,60 @@
+import 'package:auto_gpt_flutter_client/models/task_request_body.dart';
+import 'package:auto_gpt_flutter_client/utils/rest_api_utility.dart';
+
+/// Service class for performing task-related operations.
+class TaskService {
+  final RestApiUtility api;
+
+  TaskService(this.api);
+
+  /// Creates a new task.
+  ///
+  /// [taskRequestBody] is a Map representing the request body for creating a task.
+  Future<Map<String, dynamic>> createTask(
+      TaskRequestBody taskRequestBody) async {
+    try {
+      return await api.post('agent/tasks', taskRequestBody.toJson());
+    } catch (e) {
+      throw Exception('Failed to create a new task: $e');
+    }
+  }
+
+  /// Lists all tasks.
+  ///
+  /// [currentPage] and [pageSize] are optional pagination parameters.
+  ///
+  Future<List<dynamic>> listAllTasks(
+      {int currentPage = 1, int pageSize = 10}) async {
+    try {
+      return await api
+          .getList('agent/tasks?current_page=$currentPage&page_size=$pageSize');
+    } catch (e) {
+      throw Exception('Failed to list all tasks: $e');
+    }
+  }
+
+  /// Gets details about a specific task.
+  ///
+  /// [taskId] is the ID of the task.
+  Future<Map<String, dynamic>> getTaskDetails(String taskId) async {
+    try {
+      return await api.get('agent/tasks/$taskId');
+    } catch (e) {
+      throw Exception('Failed to get task details: $e');
+    }
+  }
+
+  /// Lists all artifacts for a specific task.
+  ///
+  /// [taskId] is the ID of the task.
+  /// [currentPage] and [pageSize] are optional pagination parameters.
+  Future<List<dynamic>> listTaskArtifacts(String taskId,
+      {int currentPage = 1, int pageSize = 10}) async {
+    try {
+      return await api.getList(
+          'agent/tasks/$taskId/artifacts?current_page=$currentPage&page_size=$pageSize');
+    } catch (e) {
+      throw Exception('Failed to list task artifacts: $e');
+    }
+  }
+}