Refactor PFCP procedures and handle UL CL scenario and N9 scenarios

Features

• Handle UPF associations graph structure, also in PFCP procedures
• Send N4 Session Establishment/Modification/Release messages in a defined order
• Supports UL CL scenario
• Supports N9 (Multiple UPF scenario)

Refactoring

• Created functions to create PFCP IEs
• Created functions to send N4 messages
• Reduced similar/duplicate code

Trace

This trace contains a test using 3 VPP-UPFs, where one acts as UL-CL and each UPF has a N9 interface to the UL CL. The trace shows the following:

• gnbsim attach
• gnbsim detach
• gnbsim attach
• gnbsim detach

User plane was verified using ping, but ICMP packets are not in the trace. smf_new_pfcp_procedures_ulcl_scenario_twice.pcapng