feat: introduce minLength and maxLength validation for strings

[AlbertoBarba]

Introduce string validations:
6.3.1 maxlength
6.3.2 minlength

[codecov]

Codecov Report

Patch coverage: 96.15% and project coverage change: +0.52% 🎉

Comparison is base (1ca1837) 78.14% compared to head (aa887ab) 78.67%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #111      +/-   ##
==========================================
+ Coverage   78.14%   78.67%   +0.52%     
==========================================
  Files          10       10              
  Lines        1446     1496      +50     
==========================================
+ Hits         1130     1177      +47     
- Misses        244      246       +2     
- Partials       72       73       +1     

Files Changed	Coverage Δ
pkg/generator/validator.go	94.33% <90.90%> (-0.90%)	⬇️
pkg/generator/generate.go	76.18% <100.00%> (+0.48%)	⬆️

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