GraphQL: Handle properly keys for pointer fields

[Moumouls]

Detect custom key on the child object

[codecov]

Codecov Report

Merging #6499 into master will increase coverage by 0.06%.
The diff coverage is 96.66%.

@@            Coverage Diff             @@
##           master    #6499      +/-   ##
==========================================
+ Coverage   93.92%   93.99%   +0.06%     
==========================================
  Files         169      169              
  Lines       11808    11836      +28     
==========================================
+ Hits        11091    11125      +34     
+ Misses        717      711       -6     

Impacted Files	Coverage Δ
src/GraphQL/loaders/defaultRelaySchema.js	94.73% <ø> (-0.27%)	⬇️
src/GraphQL/loaders/parseClassMutations.js	100.00% <ø> (+1.03%)	⬆️
src/GraphQL/helpers/objectsQueries.js	90.83% <93.75%> (+0.08%)	⬆️
src/GraphQL/ParseGraphQLSchema.js	97.40% <100.00%> (+0.15%)	⬆️
src/GraphQL/loaders/parseClassQueries.js	97.95% <100.00%> (ø)
src/RestWrite.js	93.81% <0.00%> (+0.32%)	⬆️
src/Adapters/Auth/apple.js	100.00% <0.00%> (+12.90%)	⬆️

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[davimacedo]

LGTM!