tls: add optional server certificate verification

[KernelClint]

TLSClientAutomaton completes a handshake and sends application data without checking the server's
certificate against a trust store or the hostname it asked for. It is a test client, and it does
not claim otherwise — but there is no way to ask it to check, which makes it awkward to use against
a real service where that is the point.

The relevant states pass the certificate through without a policy:
scapy/layers/tls/automaton_cli.py:118-142
takes the server name but has no trust store, and :396-405 advances past a TLS 1.2 Certificate
message without authenticating it.

The change adds verification and makes it selectable. When enabled, an explicit CA bundle is used if
one is supplied and the system trust store otherwise, the requested hostname is checked, and the
connection closes on failure. verify=False keeps the current behaviour for the cases where
talking to an untrusted server is the whole point of the exercise.

The added regressions cover a self-signed server, a valid certificate for the wrong hostname, and a
correct server, asserting the first two close the connection and the third completes. Without the
source change they fail.

On cost. Performance was measured on one computer, before and after the fix, but there is not
much of a "before" to measure: the client performs no verification today, so its valid path does
nothing. Verifying took 2.9 µs per handshake, against 819 ns of test overhead — roughly 2.1 µs
added, once per connection, for a full chain and hostname check. A percentage against a path that
does no work would not mean anything, so none is quoted.

[codecov]

Codecov Report

❌ Patch coverage is 81.19658% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.35%. Comparing base (e4742a5) to head (e591fc1).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/layers/tls/cert.py	78.57%	12 Missing ⚠️
scapy/layers/tls/automaton_cli.py	76.92%	9 Missing ⚠️
scapy/layers/tls/automaton_srv.py	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5144      +/-   ##
==========================================
- Coverage   80.92%   80.35%   -0.58%     
==========================================
  Files         393      393              
  Lines       98107    98197      +90     
==========================================
- Hits        79392    78902     -490     
- Misses      18715    19295     +580     

Files with missing lines	Coverage Δ
scapy/layers/tls/automaton.py	86.33% <100.00%> (-2.80%)	⬇️
scapy/layers/tls/handshake.py	88.30% <100.00%> (-0.46%)	⬇️
scapy/layers/tls/handshake_sslv2.py	93.58% <100.00%> (ø)
scapy/layers/tls/session.py	86.05% <100.00%> (+0.01%)	⬆️
scapy/layers/tls/automaton_srv.py	72.51% <92.85%> (-0.56%)	⬇️
scapy/layers/tls/automaton_cli.py	85.80% <76.92%> (-1.19%)	⬇️
scapy/layers/tls/cert.py	78.20% <78.57%> (+0.03%)	⬆️

... and 39 files with indirect coverage changes

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[gpotter2]

It also seems the test against google.com fails to verify the certificates :(

[gpotter2]

Sorry, I still have comments :( Thanks again for the work and sorry about that