Tag release for read only fields in dart

[AaronZyLee]

Description of changes

Add read only fields in dart modelgen

Issue #, if available

Description of how you validated changes

Checklist

• PR description included
• 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 #264 (87064c5) into tagged-release/custom-type-preview (d15504d) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@                          Coverage Diff                           @@
##           tagged-release/custom-type-preview     #264      +/-   ##
======================================================================
+ Coverage                               85.70%   85.71%   +0.01%     
======================================================================
  Files                                     145      145              
  Lines                                    6701     6708       +7     
  Branches                                 1543     1575      +32     
======================================================================
+ Hits                                     5743     5750       +7     
- Misses                                    877      884       +7     
+ Partials                                   81       74       -7     

Impacted Files	Coverage Δ
...nc-modelgen-plugin/src/visitors/appsync-visitor.ts	95.72% <ø> (+0.30%)	⬆️
...delgen-plugin/src/visitors/appsync-dart-visitor.ts	97.03% <100.00%> (+0.03%)	⬆️
...elgen-plugin/src/visitors/appsync-swift-visitor.ts	95.43% <0.00%> (-0.34%)	⬇️
...kages/graphql-types-generator/src/swift/helpers.ts	94.06% <0.00%> (ø)
...c-modelgen-plugin/src/utils/process-connections.ts	92.13% <0.00%> (ø)
...odelgen-plugin/src/utils/process-connections-v2.ts	90.32% <0.00%> (ø)
...delgen-plugin/src/visitors/appsync-java-visitor.ts	93.02% <0.00%> (ø)
...ugin/src/languages/typescript-declaration-block.ts	72.38% <0.00%> (ø)
...tor/src/compiler/visitors/collectAndMergeFields.ts	92.45% <0.00%> (+0.14%)	⬆️

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