[REQ] [dart-dio] Further improvements to polymorphism support

I made this issue to track what improvements can be introduced to the newly added polymorphism support for dart-dio https://github.com/OpenAPITools/openapi-generator/pull/12295
Suggestions and help are welcome !

• Merge all test schemas into one test schema for better maintainability
  • modules/openapi-generator/src/test/resources/3_0/oneof_polymorphism_and_inheritance.yaml
  • modules/openapi-generator/src/test/resources/3_0/oneOf_primitive.yaml
  • modules/openapi-generator/src/test/resources/3_0/oneOf.yaml
  • should be merged with: modules/openapi-generator/src/test/resources/3_0/petstore-with-fake-endpoints-models-for-testing.yaml
  • into a new test schema specific for dart
• Add polymorphism support to json_serializable
• Add a new QoL method inside generated classes called createConcreteBuilder for built_value serializer
  • so for example if you have class Apple, the signature would be:
    static AppleBuilder createConcreteBuilder() => AppleBuilder();
  • if you have class Fruit which class Apple inherits from, it would have the signature:
    static $FruitBuilder createConcreteBuilder() => $FruitBuilder();
• Implement MonoRepo structure for https://github.com/Bdaya-Dev/dart-oneof and write CD to auto deploy the package to pub.dev on each new release
• Auto assign discriminator type on serialization based on type (PR https://github.com/OpenAPITools/openapi-generator/pull/14291)
  @kuhnroyal @agilob @jaumard @josh-burton @amondnet @sbu-WBT @banool

Review the dart-dio polymorphism support introduced by PR #12295 and the listed schemas under modules/openapi-generator/src/test/resources/3_0/. Choose one unchecked item and first run the related dart-dio tests, if available. Done means the selected improvement is implemented with updated schema or generated-code coverage; the issue does not define a single acceptance scope for all items.