Add LEB128 variable-lengh integer support to cstruct v3

[joost-j]

Backport for #69. Basically copy-pasted the implementation from the v4 version, but slightly edited some implementation / test details to work with the cstruct v3 API.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.44444% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 89.50%. Comparing base (772d473) to head (98c3553).

Files	Patch %	Lines
dissect/cstruct/types/leb128.py	94.23%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #73      +/-   ##
==========================================
+ Coverage   89.09%   89.50%   +0.41%     
==========================================
  Files          20       21       +1     
  Lines        1870     1924      +54     
==========================================
+ Hits         1666     1722      +56     
+ Misses        204      202       -2     

Flag	Coverage Δ
unittests	89.50% <94.44%> (+0.41%)	⬆️

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