refactor: initialize validator and translator once at package level

[bfabio]

The validator is thread safe and expensive to create (it registers all custom validators and translations).

Build it once at package initialization.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.42%. Comparing base (58b8033) to head (93a3fb9).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
fields.go	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #313      +/-   ##
==========================================
+ Coverage   45.35%   50.42%   +5.06%     
==========================================
  Files          14       14              
  Lines        1087     1188     +101     
==========================================
+ Hits          493      599     +106     
+ Misses        563      557       -6     
- Partials       31       32       +1     

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