Improved Release of the P4sim Module for ns-3

Description:
This release includes significant enhancements to the P4sim module, making it more user-friendly and suitable for simulating programmable switches in ns-3.

Highlights:

1. Added an automatic script to generate fat-tree topologies based on a user-defined K value.
2. Automatically generates table entries (IPv4 forwarding and ARP) for P4 scripts based on the topology, simplifying the simulation setup. See example/p4-topo-fattree for details.
3. Improved example use cases and added support for the artifact repository: P4sim Artifact: Simulating Programmable Switches in ns-3.
4. Apache-2.0 license has been added for open-source compliance.

Release for a better modular management

Here are some changes:

• Restructure the code updata the p4-switch-core as the parent class of p4 switch core
• add license for the code
• Build with ns-3.39 C++17
• Merge branch 'waf'
• Perforamce
• add primitives for v1model
• Add more examples. Add load balance example

v0.1.0-custom-header

Add Custom Header.
Core modification: custom-header, custom-p2p-net-device
PS: The p4-sim can simulate the custom-defined header, check the example p4-custom-test.cc, which implements the Tunnel for the forwarding, without that tunnel, the flow will go through all the switches, with the tunnel, it will only pass the switch 0 and 1, in that case, shorten the flow path. Now we also support the CSMA channel and P2P channel by P4GlobalVar::g_channelType.