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>

CMakeLists.txt

@@ -363,6 +363,7 @@ add_library(ngap
   ${NGAP_DIR}/ngap_gNB.c
   ${NGAP_DIR}/ngap_common.c
   ${NGAP_DIR}/ngap_gNB_context_management_procedures.c
+  ${NGAP_DIR}/ngap_gNB_mobility_management.c
   ${NGAP_DIR}/ngap_gNB_decoder.c
   ${NGAP_DIR}/ngap_gNB_encoder.c
   ${NGAP_DIR}/ngap_gNB_handlers.c