This commit completes the NG Mobility Management Procedure known as Handover Resource Allocation. The outbound message is sent by the target NG-RAN and is known as Handover Request Acknowledge. Major changes: * add RRC callback to trigger the generation of NG Handover Request Acknowledge on the target NG-RAN * add NG Handover Request Acknowledge encoder Co-authored-by: batuhan duyuler <batuhan.duyuler@firecell.io>