feat: custom linked list api definition

assignment/src/main/java/appointmentplanner/customlist/api/CustomLinkedList.java

@@ -0,0 +1,21 @@
+
+package appointmentplanner.customlist.api;
+
+public interface CustomLinkedList<T> extends Iterable<T> {
+
+  void add(T item);
+
+  void remove(T item);
+
+  void insertAfter(T reference, T item);
+
+  void insertBefore(T reference, T item);
+
+  T getAfter(T reference);
+
+  T getBefore(T reference);
+
+  boolean contains(T item);
+
+  int size();
+}