[REQ] [dart-dio] Refactor api methods and classes

Is your feature request related to a problem? Please describe.

currently dart-dio

1. generates api methods that handle both serialization and networking (see https://github.com/OpenAPITools/openapi-generator/issues/15427#issuecomment-1540251921 for further explaination)
2. generates api classes are tightly coupled to the serializer, which means that different serialization options have to change networking mustache files directly, leading to complexities mentioned in https://github.com/OpenAPITools/openapi-generator/issues/15449#issuecomment-1539943367
   e.g.:
   • json_serializable: https://github.com/OpenAPITools/openapi-generator/tree/master/modules/openapi-generator/src/main/resources/dart/libraries/dio/serialization/json_serializable/api
   • built_value: https://github.com/OpenAPITools/openapi-generator/tree/master/modules/openapi-generator/src/main/resources/dart/libraries/dio/serialization/built_value/api

Describe the solution you'd like

We can mitigate both of these issues with some non-breaking refactor of the generated API classes, essentially removing that api mustache folder, that each serializer has to adapt.

• for the first issue, we introduce 2 method signatures (raw + json, as described in my comment)
• for the second issue, we introduce a new interface IJsonSerializationRepository which all serialization options have to implement.
  It will include these 2 abstract methods

  Object serialize<T>(T src, {Object? context});
  T deserialize<T>(Object value, {Object? context});
  

  where the general rule is that

  json == serialize<T>(deserialize<T>(json))
  object == deserialize<T>(serialize<T>(object))
  

Start by reading the referenced issue comments, then inspect the dart-dio templates under modules/openapi-generator/src/main/resources/dart/libraries/dio/serialization/, especially the json_serializable/api and built_value/api directories. Trace how generated API methods currently combine serialization and networking. Done means the proposed raw and JSON method signatures and shared IJsonSerializationRepository design are reflected consistently across the serialization options.