fix(predictions): use string's unicode scalars view to compute indexes for Comprehend results

[diegocstn]

Amazon Comprehend returns an array of Entity https://docs.aws.amazon.com/comprehend/latest/dg/API_Entity.html .
Each entity has a beginOffset and endOffset properties, which are positions of each UTF-8 "code point" in the string.

A code point is a value in the Unicode codespace, the range of integers from 0 to 10FFFF, not all code points are assigned to encoded chars.
We need to use Swift strings underlying scalar view to compute indexes correctly and avoid crashes at runtime.

fix #873

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 #904 (af4b017) into main (a0d37bb) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #904      +/-   ##
==========================================
+ Coverage   66.87%   66.89%   +0.01%     
==========================================
  Files         874      874              
  Lines       34755    34755              
==========================================
+ Hits        23244    23248       +4     
+ Misses      11511    11507       -4     

Flag	Coverage Δ
API_plugin_unit_test	62.56% <ø> (+0.07%)	⬆️
Analytics_plugin_unit_test	72.38% <ø> (ø)
Auth_plugin_unit_test	39.11% <ø> (ø)
DataStore_plugin_unit_test	83.03% <ø> (ø)
Predictions_plugin_unit_test	49.49% <100.00%> (ø)
Storage_plugin_unit_test	74.74% <ø> (ø)
build_test_amplify	63.16% <ø> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
...Predictions/AWSPredictionsService+Comprehend.swift	96.47% <100.00%> (ø)
...Hub/DefaultPluginTests/DefaultHubPluginTests.swift	88.37% <0.00%> (+1.16%)	⬆️
...s/AWSHubPlugin/Internal/HubChannelDispatcher.swift	97.05% <0.00%> (+2.94%)	⬆️
...n/Operation/AWSGraphQLOperation+APIOperation.swift	54.23% <0.00%> (+3.38%)	⬆️

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[royjit]

LGTM