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>