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

[ahmednfwela]

I made this issue to track what improvements can be introduced to the newly added polymorphism support for dart-dio #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 [dart-dio] Fix non legacy discriminator behavior #14291)
  @kuhnroyal @agilob @jaumard @josh-burton @amondnet @sbu-WBT @banool

[banool]

I would love for the generator to generate code that adds OneOf support in a type-checkable way. Currently using the OneOf stuff is quite hard to get right and the only way you can iterate is at runtime. For example, if you have a struct my MyOneOfStruct and it has a oneof field that expects A or B, you can pass in C and it will not complain at type check time, but fail at runtime. This makes it a real nightmare to know what to use.

In a similar vein, I'd love if the generatated code hid the types that you shouldn't need to use as an end user, such as the abstract classes, the *AllOf types, etc. These also make it super confusing what you're meant to use (not to mention all the $, $$, _$, etc. classes.

[ahmednfwela]

to add oneOf type check support, you would need the language to have union types dart-lang/language#1222

[ahmednfwela]

the abstract classes are intended to be used by the end user

the QoL function I am suggesting is to make the user not need the $ classes