[FEATURE] Add mocked UPF to aio5gc to test PacketRusher's GTP-U stack

[linouxis9]

Is your feature request related to a problem? Please describe.
Our All-in-One 5G Core that is used for integration testing of PacketRusher in the CI already supports the creation and establishment of PDU Sessions.
The next step is to be able to also use this PDU Session with data traffic in integration tests, by adding a GTP-U stack inside aio5gc.

• Goal: Validate that user traffic can flow between the UE and Data Network in CI.
• Non-goal: The goal is not to do performance regression testing for now.
  • We might add an high performance open source UPF in CI for that

Describe the solution you'd like
Use https://github.com/wmnsk/go-gtp

Describe alternatives you've considered
Use the eBPF stack used by PacketRusher. I would advise against it as we may have conflicts between the PacketRusher's eBPF and aio5gc's eBPF.

Additional context
N/A