Add NGAP Handover Notify encoder and trigger on the target NG-RAN

This commit introduces the NG Mobility Management Procedure known as
Handover Notification. The outbound message is sent by the target NG-RAN
and is known as Handover Notify. The procedure is used to indicate to
the AMF that the UE has arrived to the target cell and the NG-based
handover has been successfully completed

* after RRCReconfiguration complete, in the N2 callback for HO success
* add NGAP encoder for the message
* RRC callback to trigger Handover Notify message

Co-authored-by: batuhan duyuler <batuhan.duyuler@firecell.io>

openair3/NGAP/ngap_gNB_encoder.c

@@ -53,7 +53,8 @@ static inline int ngap_gNB_encode_initiating(NGAP_NGAP_PDU_t *pdu, uint8_t **buf
                                       NGAP_ProcedureCode_id_UEContextReleaseRequest,
                                       NGAP_ProcedureCode_id_PathSwitchRequest,
                                       NGAP_ProcedureCode_id_PDUSessionResourceModifyIndication,
-                                      NGAP_ProcedureCode_id_HandoverPreparation};
+                                      NGAP_ProcedureCode_id_HandoverPreparation,
+                                      NGAP_ProcedureCode_id_HandoverNotification};
   int i;
   for (i = 0; i < sizeofArray(tmp); i++)
     if (pdu->choice.initiatingMessage->procedureCode == tmp[i])