fix(modelgen-swift): respect index sk fields in associate fields

[AaronZyLee]

Description of changes

• The associatedWith fields now respect the index sortKeyFields in explicit uni hasMany relation. This change only affects swift modelgen.
• Rewrite the flag value parsing for processDirectives. Add new type definition and default value for directive process configuration.

Issue #, if available

Fix #539

Description of how you validated changes

yarn test amplify-dev codegen models

Checklist

• [x] PR description included
• [x] yarn test passes
• [ ] Tests are changed or added
• [ ] Relevant documentation is changed or added (and PR referenced)
• [ ] Breaking changes to existing customers are released behind a feature flag or major version update
• [ ] Changes are tested using sample applications for all relevant platforms (iOS/android/flutter/Javascript) that use the feature added/modified

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

[codecov-commenter]

Codecov Report

Merging #542 (f0bc596) into main (181480d) will increase coverage by 0.02%. The diff coverage is 96.55%.

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@@            Coverage Diff             @@
##             main     #542      +/-   ##
==========================================
+ Coverage   85.69%   85.71%   +0.02%     
==========================================
  Files         148      148              
  Lines        7380     7364      -16     
  Branches     1962     1958       -4     
==========================================
- Hits         6324     6312      -12     
+ Misses        959      955       -4     
  Partials       97       97              

Impacted Files	Coverage Δ
...-plugin/src/visitors/appsync-typescript-visitor.ts	82.89% <0.00%> (+1.60%)	:arrow_up:
...nc-modelgen-plugin/src/utils/process-belongs-to.ts	94.11% <100.00%> (+0.17%)	:arrow_up:
...odelgen-plugin/src/utils/process-connections-v2.ts	92.20% <100.00%> (-0.20%)	:arrow_down:
...sync-modelgen-plugin/src/utils/process-has-many.ts	90.80% <100.00%> (+0.44%)	:arrow_up:
...psync-modelgen-plugin/src/utils/process-has-one.ts	96.15% <100.00%> (+3.84%)	:arrow_up:
...delgen-plugin/src/visitors/appsync-dart-visitor.ts	98.11% <100.00%> (-0.01%)	:arrow_down:
...delgen-plugin/src/visitors/appsync-java-visitor.ts	94.78% <100.00%> (-0.03%)	:arrow_down:
...-plugin/src/visitors/appsync-javascript-visitor.ts	97.72% <100.00%> (-0.15%)	:arrow_down:
...ugin/src/visitors/appsync-json-metadata-visitor.ts	95.74% <100.00%> (-0.14%)	:arrow_down:
...rc/visitors/appsync-model-introspection-visitor.ts	98.66% <100.00%> (-0.06%)	:arrow_down:
... and 2 more

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