Add NGAP Handover Required message NGAP encoder and RRC callback on source NG-RAN

This commit introduces the NG Mobility Management Procedure known as
Handover Preparation. The outbound message is sent by the source NG-RAN
and is known as Handover Required.

* introduce NGAP library for Mobility Management
* introduce NGAP IEs encoder functions for Handover Required and relevant common IEs
* handle RRC trigger in NGAP and Send NGAP Handover Required to the AMF

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

openair3/NGAP/ngap_gNB_encoder.c

@@ -52,7 +52,8 @@ static inline int ngap_gNB_encode_initiating(NGAP_NGAP_PDU_t *pdu, uint8_t **buf
                                       NGAP_ProcedureCode_id_NASNonDeliveryIndication,
                                       NGAP_ProcedureCode_id_UEContextReleaseRequest,
                                       NGAP_ProcedureCode_id_PathSwitchRequest,
-                                      NGAP_ProcedureCode_id_PDUSessionResourceModifyIndication};
+                                      NGAP_ProcedureCode_id_PDUSessionResourceModifyIndication,
+                                      NGAP_ProcedureCode_id_HandoverPreparation};
   int i;
   for (i = 0; i < sizeofArray(tmp); i++)
     if (pdu->choice.initiatingMessage->procedureCode == tmp[i])