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_msg_includes.h

@@ -101,5 +101,6 @@
 #include "NGAP_HandoverRequestAcknowledgeTransfer.h"
 #include "NGAP_QosFlowItemWithDataForwarding.h"
 #include "NR_HandoverPreparationInformation.h"
+#include "NGAP_HandoverNotify.h"
 
 #endif // NGAP_MSG_INCLUDES_H