Add function to know if a field is required

[PhiPhi42]

Can be usefull when you want to put a star (*) on required fields

[codecov-io]

Codecov Report

Merging #175 into master will decrease coverage by 0.26%.
The diff coverage is 25%.

@@            Coverage Diff             @@
##           master     #175      +/-   ##
==========================================
- Coverage   94.21%   93.94%   -0.27%     
==========================================
  Files          24       24              
  Lines        2090     2096       +6     
  Branches      232      234       +2     
==========================================
  Hits         1969     1969              
- Misses        107      113       +6     
  Partials       14       14

Impacted Files	Coverage Δ
...ssiveAnnotations/Attributes/RequiredIfAttribute.cs	61.29% <25%> (-14.71%)	⬇️

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