Refactor OppdragRequest: Change String to KodeEndring enum

Changed the type of `kodeEndring` from String to a new KodeEndring enum to improve type safety and reduce potential errors. Introduced the new KodeEndring enum with values NY, ENDR, and UEND.

libs/data-transfer-objects/src/main/java/no/nav/testnav/libs/dto/oppdragservice/v1/OppdragRequest.java

@@ -28,7 +28,7 @@ public class OppdragRequest {
     private Ompostering ompostering;
     private List<Oppdragslinje> oppdragslinje;
     @NotBlank
-    private String kodeEndring;
+    private KodeEndring kodeEndring;
     private KodeStatus kodeStatus;
     private String datoStatusFom;
     @NotBlank
@@ -336,6 +336,12 @@ public enum KodeStatus {
         FEIL;
     }
 
+    public enum KodeEndring {
+        NY,
+        ENDR,
+        UEND
+    }
+
     @Data
     @Builder
     @NoArgsConstructor