feat/Created new NotificationType enum class

src/main/java/no/ntnu/idi/stud/savingsapp/model/notification/NotificationType.java

+package no.ntnu.idi.stud.savingsapp.model.notification;
+
+/**
+ * Enum representing the type associated with a notification.
+ */
+public enum NotificationType {
+
+  BADGE,
+  FRIEND_REQUEST,
+  COMPLETED_GOAL
+
+}