TF-123 Create objects for caching mailbox

model/lib/extensions/properties_extension.dart

@@ -0,0 +1,7 @@
+
+import 'package:jmap_dart_client/jmap/core/properties/properties.dart';
+
+extension PropertiesExtension on Properties {
+
+  bool contain(String key) => value.contains(key);
+}