Add AI related env var

AI_ENABLED allows to test AI features only in some wanted environment.

AI_API_KEY and AI_API_URL allows to call the LLM while we wait for tmail backend endpoints.
It will be removed when tmail backend endpoints are ready.

lib/main/utils/app_config.dart

@@ -77,4 +77,10 @@ class AppConfig {
   static bool get isCozyIntegrationEnabled => dotenv.get('COZY_INTEGRATION', fallback: 'false') == 'true';
 
   static String get cozyExternalBridgeVersion => dotenv.get('COZY_EXTERNAL_BRIDGE_VERSION', fallback: '0.16.1');
+
+  static bool get isAiEnabled => dotenv.get('AI_ENABLED', fallback: 'false') == 'true';
+
+  static String get aiApiKey => dotenv.get('AI_API_KEY', fallback: '');
+ 
+  static String get aiApiUrl => dotenv.get('AI_API_URL', fallback: '');
 }