Add Tungsten Fabric VRouter agent protocol

[myaut]

This Pull Request adds scapy.contrib.tf_agent module which can be used for pcaps captured on pkt0 interface (it is used to punt packets from Tungsten Fabric VRouter to user-space agent, but has special encapsulation header)

More info on how pkt0 works is here: https://tungstenfabric.github.io/website/Tungsten-Fabric-Architecture.html#packet-processing

Usage example:

#!/usr/bin/python3
from scapy.all import rdpcap
import scapy.contrib.tf_agent

pcap = rdpcap("pkt0.pcap")
for pkt in pcap:
    print(pkt[1].mysummary(), "|", pkt[2].summary())

[codecov[bot]]

Codecov Report

Merging #2509 into master will decrease coverage by 5.46%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2509      +/-   ##
==========================================
- Coverage   87.76%   82.29%   -5.47%     
==========================================
  Files         242      136     -106     
  Lines       51071    34548   -16523     
==========================================
- Hits        44824    28433   -16391     
+ Misses       6247     6115     -132

Impacted Files	Coverage Δ
scapy/layers/tftp.py	44.54% <0%> (-45.12%)	:arrow_down:
scapy/contrib/http2.py	61.24% <0%> (-34.11%)	:arrow_down:
scapy/layers/usb.py	49.47% <0%> (-31.58%)	:arrow_down:
scapy/supersocket.py	52.06% <0%> (-22.54%)	:arrow_down:
scapy/consts.py	70.83% <0%> (-20.84%)	:arrow_down:
scapy/contrib/gtp.py	69.09% <0%> (-20%)	:arrow_down:
scapy/arch/pcapdnet.py	61.96% <0%> (-18.44%)	:arrow_down:
scapy/layers/tls/handshake_sslv2.py	76.49% <0%> (-13.81%)	:arrow_down:
scapy/arch/__init__.py	84.44% <0%> (-11.12%)	:arrow_down:
scapy/layers/l2.py	76.06% <0%> (-9.72%)	:arrow_down:
... and 144 more