feat: Flutter json_serializable use JsonValue as the json key of Enum

[whlsxl]

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Description

Related #713

Type of change

Please delete options that are not relevant.

• Breaking change (fix or feature that would cause existing functionality to not work as
  expected)

Screenshots/Sandbox (if appropriate/relevant):

https://stackblitz.com/edit/github-mc3hpg?file=app_models.dart

How Has This Been Tested?

Change the test in https://github.com/whlsxl/graphql-code-generator-community/blob/main/packages/plugins/dart/flutter-freezed/tests/plugin.spec.ts. to verify this pull request.

Test Environment:

• OS: macOS 14.0

•   "@graphql-codegen/cli": "5.0.2",
    "@graphql-codegen/introspection": "4.0.3",
    "@graphql-codegen/typescript": "4.0.6",
    "@graphql-codegen/typescript-resolvers": "4.0.6",```
  

• NodeJS: v21.7.2

Checklist:

• I have followed the
  CONTRIBUTING doc and the
  style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Further comments

From json_serializable version 6, Enum json key from JsonKey to JsonValue, this pull request temporarily solve this problem, but requires more systematic modifications.

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