Add NGAP Handover Request decoding and RRC handler

This commit introduces the NG Mobility Management Procedure known as
Handover Resource Allocation. The inbound message is sent by the AMF
and is known as Handover Request.

Major changes:

(0) Add Handover Resource Allocation initiating message decoding
    * add case for NGAP initiating message and unsuccessful message decoding
(1) handle Handover Request on the target NG-RAN
(2) decode NG Handover Request message
(3) process payload, e.g. create UE context,
    store IDs, set UP security and trigger bearer setup

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

openair3/NGAP/ngap_gNB_decoder.c

@@ -94,6 +94,12 @@ static int ngap_gNB_decode_initiating_message(NGAP_NGAP_PDU_t *pdu) {
       NGAP_INFO("TODO ErrorIndication initiating message\n");
       break;
 
+    case NGAP_ProcedureCode_id_HandoverResourceAllocation:
+      res = asn_encode_to_new_buffer(NULL, ATS_CANONICAL_XER, &asn_DEF_NGAP_NGAP_PDU, pdu);
+      free(res.buffer);
+      NGAP_INFO("Handover Resource Allocation initiating message\n");
+      break;
+
     default:
       NGAP_ERROR("Unknown procedure ID (%d) for initiating message\n",
                  (int)pdu->choice.initiatingMessage->procedureCode);