Preliminary work for PKINIT support

[gpotter2]

This PR does preliminary work to add PKINIT support to the Kerberos client.

• add CMS ASN.1 structures
• add PKINIT structures
• add some other ASN.1 structures
• templating of the client / parameters to add preliminary support
• minor changes to prepare for PKCS12 loading in PrivKey-related classes

The tests are from running a MIT kerberos kinit client against a dummy domain.

[codecov]

Codecov Report

❌ Patch coverage is 72.41379% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.88%. Comparing base (5eb00ba) to head (2becb4b).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/layers/kerberos.py	43.18%	25 Missing ⚠️
scapy/asn1fields.py	80.00%	2 Missing ⚠️
scapy/layers/tls/cert.py	75.00%	2 Missing ⚠️
scapy/layers/x509.py	95.45%	2 Missing ⚠️
scapy/asn1/asn1.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4840      +/-   ##
==========================================
+ Coverage   80.60%   80.88%   +0.27%     
==========================================
  Files         368      368              
  Lines       90244    90340      +96     
==========================================
+ Hits        72741    73067     +326     
+ Misses      17503    17273     -230     

Files with missing lines	Coverage Δ
scapy/asn1/mib.py	91.61% <100.00%> (+0.22%)	⬆️
scapy/asn1/asn1.py	86.40% <83.33%> (ø)
scapy/asn1fields.py	83.71% <80.00%> (+0.09%)	⬆️
scapy/layers/tls/cert.py	83.26% <75.00%> (+0.24%)	⬆️
scapy/layers/x509.py	97.56% <95.45%> (-0.24%)	⬇️
scapy/layers/kerberos.py	60.81% <43.18%> (-0.25%)	⬇️

... and 10 files with indirect coverage changes

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