add functionality for group_ids to User class

Co-Authored-By: tomvahl <tomvahl@users.noreply.github.com>

src/main/java/mops/gruppen2/domain/User.java

@@ -5,6 +5,8 @@ import lombok.Data;
 import lombok.EqualsAndHashCode;
 import lombok.Value;
 
+import java.util.List;
+
 @Value
 @AllArgsConstructor
 @EqualsAndHashCode(exclude = {"givenname", "familyname", "email"})
@@ -15,4 +17,10 @@ public class User {
     String givenname;
     String familyname;
     String email;
+
+    List<Long> group_ids;
+
+    public void addGroup(Long group_id){
+        group_ids.add(group_id);
+    }
 }