ForwardingMachine + Cert() overhaul

[gpotter2]

• new: ForwardingMachine. A tool that allows to forward traffic while modifying it with the power of Scapy (in both SERVER or TPROXY mode)
• comes with doc
• rework the "advanced usage" doc by splitting it
• improve the Cert() and Key() classes in TLS to be more user friendly, and generally be more intuitive

[codecov]

Codecov Report

Attention: Patch coverage is 22.54335% with 268 lines in your changes missing coverage. Please review.

Project coverage is 81.05%. Comparing base (50edac9) to head (729993a).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/fwdmachine.py	0.00%	242 Missing ⚠️
scapy/layers/tls/cert.py	73.68%	25 Missing ⚠️
scapy/layers/tls/keyexchange_tls13.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4784      +/-   ##
==========================================
- Coverage   81.28%   81.05%   -0.24%     
==========================================
  Files         364      365       +1     
  Lines       88691    88968     +277     
==========================================
+ Hits        72094    72110      +16     
- Misses      16597    16858     +261     

Files with missing lines	Coverage Δ
scapy/config.py	80.58% <100.00%> (+0.02%)	⬆️
scapy/fields.py	92.88% <100.00%> (ø)
scapy/supersocket.py	75.92% <100.00%> (+0.06%)	⬆️
scapy/layers/tls/keyexchange_tls13.py	79.89% <50.00%> (ø)
scapy/layers/tls/cert.py	84.15% <73.68%> (-2.01%)	⬇️
scapy/fwdmachine.py	0.00% <0.00%> (ø)

... and 3 files with indirect coverage changes

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