Fill default keys from OpenFeature Schema + Removed requirement for `valueType` in **FeatureBuilder**

[codecov[bot]]

Codecov Report

Merging #72 (a752fad) into develop (6e16dce) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop      #72      +/-   ##
===========================================
+ Coverage    97.74%   97.76%   +0.01%     
===========================================
  Files           81       81              
  Lines         1510     1521      +11     
===========================================
+ Hits          1476     1487      +11     
  Misses          34       34              

Impacted Files	Coverage Δ
...ts/feature_provider/engine/open_feature_flags.dart	100.00% <ø> (ø)
example/lib/asset_feature_provider.dart	100.00% <100.00%> (ø)
example/lib/home_page.dart	100.00% <100.00%> (ø)
...faults/feature_provider/json_feature_provider.dart	100.00% <100.00%> (ø)
lib/src/open_feature/src/core/features.dart	100.00% <100.00%> (ø)
lib/src/widgets/src/feature_builder.dart	100.00% <100.00%> (ø)

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