[Dart-Dio-Next]#9082 Add json_serializable serialization option

Created by: MichaelMarner

This is a WIP/PoC to add json_serializable as an alternative to built_value in the Dart-Dio-Next generator.

For context: We would like to migrate our Flutter app to full null safety, and so we are migrating from the old Dart generator to Dart-Dio-Next.

We have found that moving from models with to/fromJson methods to using built_value has made the job of migrating bigger and riskier than we'd like. We need to rewrite large chunks of our existing persistence layer and other parts.

Adding json_serializable as an option has mostly involved reworking existing templates in the old Dart generator to generate null safe code.

Fixes #9082 (closed)

@swipesight @jaumard @josh-burton @amondnet @sbu-WBT @kuhnroyal @agilob

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