feat: setup api key for sudoku backend

.env_example

@@ -1 +1,2 @@
 API_BASE_URL = <API_BASE_URL>
+API_KEY = <YOUR_API_KEY>

lib/api/dio/sudoku_dio_client.dart

@@ -2,6 +2,7 @@ import 'package:dio/dio.dart';
 import 'package:flutter/foundation.dart';
 import 'package:sudoku/api/api.dart';
 import 'package:sudoku/api/dtos/dtos.dart';
+import 'package:sudoku/env/env.dart';
 import 'package:sudoku/models/difficulty.dart';
 import 'package:sudoku/models/sudoku.dart';
 
@@ -20,6 +21,10 @@ class SudokuDioClient extends SudokuAPI {
   @visibleForTesting
   final Dio dioClient;
 
+  Map<String, String> get _headers => {
+    'x-api-key': Env.apiKey,
+  };
+
   /// HTTP request path for creating sudoku
   static const createSudokuPath = '/createSudokuFlow';
 
@@ -33,6 +38,7 @@ class SudokuDioClient extends SudokuAPI {
         ).toJson(),
         options: Options(
           contentType: Headers.jsonContentType,
+          headers: _headers,
         ),
       );
 

lib/env/env.dart

@@ -6,4 +6,7 @@ part 'env.g.dart';
 abstract class Env {
   @EnviedField(varName: 'API_BASE_URL')
   static final String apiBaseUrl = _Env.apiBaseUrl;
+
+  @EnviedField(varName: 'API_KEY')
+  static final String apiKey = _Env.apiKey;
 }

lib/storage/storage_api.dart

@@ -13,7 +13,7 @@ abstract class StorageAPI {
   Future<void> storePuzzle({required Puzzle puzzle});
 
   /// Provides the stored puzzle.
-  /// 
+  ///
   /// Returns null, if there is no puzzle stored.
   Future<Puzzle?> getPuzzle();
 

test/env/env_test.dart

@@ -0,0 +1,11 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:sudoku/env/env.dart';
+
+void main() {
+  group('Env', () {
+    test('sets up environment variables correctly', () {
+      expect(Env.apiBaseUrl, isA<String>());
+      expect(Env.apiKey, isA<String>());
+    });
+  });
+}