fields: decode BitLenField integers with int.from_bytes

[KernelClint]

Mpint.length controls the width of an SSH key-exchange integer at
scapy/layers/ssh.py:102-105.
_BitField.getfield() unpacks the value into individual bytes and performs one growing shift for
each at
scapy/fields.py:2432-2447,
so an SSH peer can make numeric parsing quadratic.

Median decoding grew to 2,403.01 ms at 256,000 bytes, with a 1.89 exponent and a 1.2% noise floor.
Patched decoding took 0.21 ms at 256,000 bytes and 0.83 ms at 1,024,000 bytes, with a 0.98 exponent
and an 8.1% noise floor.

This change retains the existing short-buffer struct.error and converts the complete value with
int.from_bytes(). The focused regression counted eight conversions before the patch and one
after it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.60%. Comparing base (ba8641a) to head (207d480).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5108   +/-   ##
=======================================
  Coverage   80.59%   80.60%           
=======================================
  Files         390      390           
  Lines       96892    96895    +3     
=======================================
+ Hits        78094    78099    +5     
+ Misses      18798    18796    -2     

Files with missing lines	Coverage Δ
scapy/fields.py	92.87% <100.00%> (-0.01%)	⬇️

... and 8 files with indirect coverage changes

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