fix: refactor GraphQL response decoder

[lawmicha]

Issue #, if available:

Description of changes:
This refactors the GraphQL decoder code to isolate decoding logic into decodeToResponseType() in anticipation of pagination support decoding

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Merging #971 (409357f) into main (492c0a3) will increase coverage by 0.19%.
The diff coverage is 81.87%.

@@            Coverage Diff             @@
##             main     #971      +/-   ##
==========================================
+ Coverage   54.59%   54.78%   +0.19%     
==========================================
  Files         631      634       +3     
  Lines       18371    18372       +1     
==========================================
+ Hits        10030    10066      +36     
+ Misses       8341     8306      -35     

Flag	Coverage Δ
API_plugin_unit_test	47.97% <81.87%> (+2.15%)	⬆️
AWSPluginsCore	72.41% <ø> (ø)
Amplify	45.79% <ø> (ø)
Analytics_plugin_unit_test	73.23% <ø> (ø)
Auth_plugin_unit_test	67.34% <ø> (ø)
DataStore_plugin_unit_test	71.02% <ø> (ø)
Predictions_plugin_unit_test	33.48% <ø> (ø)
Storage_plugin_unit_test	57.72% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...oryPlugin/Support/Decode/GraphQLErrorDecoder.swift	70.73% <0.00%> (ø)
...Plugin/Support/Decode/GraphQLResponseDecoder.swift	72.41% <72.41%> (ø)
...n/Support/Internal/AWSAppSyncGraphQLResponse.swift	86.00% <86.00%> (ø)
...ort/Decode/GraphQLResponseDecoder+DecodeData.swift	86.95% <86.95%> (ø)
...ICategoryPlugin/AWSAPIPlugin+GraphQLBehavior.swift	100.00% <100.00%> (ø)
...n/Operation/AWSGraphQLOperation+APIOperation.swift	47.27% <100.00%> (-3.58%)	⬇️
...CategoryPlugin/Operation/AWSGraphQLOperation.swift	60.75% <100.00%> (ø)
...in/Operation/AWSGraphQLSubscriptionOperation.swift	70.17% <100.00%> (-0.77%)	⬇️
...port/Utils/GraphQLRequest+toOperationRequest.swift	100.00% <100.00%> (ø)
... and 4 more

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[diegocstn]

nice! :)