Refactor notification service abstraction

- Removed observer map from Subject base class
- Removed notify() from Subject interface
- Made NotificationManagementService abstract with pure virtual methods
- Added static observer maps to notification service implementations
- Added attach() and detach() overrides in inventory, payment, and service management services
- Added update() override in User model for observer pattern support
- Updated notification method declarations with override specifiers

Trenser.VehicleServiceSystem/Trenser.VehicleServiceSystem/models/User.cpp

@@ -116,3 +116,7 @@ void User::setState(util::State status)
 {
     m_status = status;
 }
+
+void User::update(Notification* notification)
+{
+}

Trenser.VehicleServiceSystem/Trenser.VehicleServiceSystem/models/User.h

@@ -41,4 +41,5 @@ public:
     void addNotification(Notification* notification);
     void setRole(util::UserType role);
     void setState(util::State status);
+    void update(Notification* notification) override;
 };