Add N2 based Handover

[louisroyer]

This PR adds N2 handover procedure. You can trigger the handover using the nr-cli subcommand handover <UE-ID> <gNBTargetID> on the source gNB.

For example, to do a handover from gNB1 to gNB2:

gNB1$ nr-cli $(nr-cli --dump)                                                                                                                                                                                                                           
--------------------------------------------------------------------------------------------                                                                                                                                                                                     
$ ue-list                                                                                                                                                                                                                                                                        
- ue-id: 1                                                                                                                                                                                                                                                                       
  ran-ngap-id: 1                                                                                                                                                                                                                                                                 
  amf-ngap-id: 2                                                                                                                                                                                                                                                                 
--------------------------------------------------------------------------------------------                                                                                                                                                                                     
$ handover 1 2                                                                                                                                                                                                                                                                   
Requesting Handover procedure for UE[1] on NG-RAN-ID: 2                                                                                                                                                                                                                          
--------------------------------------------------------------------------------------------                                                                                                                                                                                     
$ ue-list                                                                                                                                                                                                                                                                        
                                                                                                                                                                                                                                                                                 
--------------------------------------------------------------------------------------------   

gNB2$ nr-cli $(nr-cli --dump)                                                                                                                                                                                                                           
--------------------------------------------------------------------------------------------
$ ue-list  
- ue-id: 2
  ran-ngap-id: 2
  amf-ngap-id: 2

We tested the N2 handover procedure using Free5GC version 4.0.1; we also found and fixed a bug in Free5GC's AMF. To test the pull request, make sure to apply this patch in your AMF first.

Data forwarding is not yet implemented, but could be added in a second time.

This project was initiated in 2023 by @mamadou-89 and was finalized by @Sid-Romero this year as part of their internships at IRIT, and I’d like to thank them again for their good work.

[aligungr]

Wow, that's a good one.

I appreciate your work @mamadou-89 @Sid-Romero @louisroyer

Would you like to provide some documentations and example usages, so that I can update the wiki page?

Also please confirm that you guys successfully tested all existing features again with and without the new handover feature. Unfortunately I am currently unable to perform the required tests.