feat: add maybeValue option on codegen options (#405)

[lorenzofaresin]

Resolve: Enhanced codegen config add maybeValue option #405

Permit to use code generation with key?: T instead key: T | null because when we define gql query without some property we expect that the undefined property doesn't exists. That's different from exists with null value

[codecov]

Codecov Report

Merging #406 (3a70a7f) into main (38f922b) will increase coverage by 0.02%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #406      +/-   ##
==========================================
+ Coverage   93.32%   93.34%   +0.02%     
==========================================
  Files           6        6              
  Lines         629      631       +2     
  Branches      120      120              
==========================================
+ Hits          587      589       +2     
  Misses         42       42              

Files	Coverage Δ
src/generate.ts	95.00% <100.00%> (+0.06%)	⬆️
src/module.ts	89.28% <100.00%> (+0.03%)	⬆️

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[Diizzayy]

@lorenzofaresin Thanks for the PR! This looks good to me