[REQ][dart] Dart full OAS3 support

This is an epic to track remaining work for full dart/dart-dio OAS3 support.

General:

• OAS3 fake petstore contains a model called File. This is generated as a separate model in the Go client but as java.io.File in Java/Kotlin. It should probably be a separate model. Currently this leads to compile errors. (addressed by https://github.com/OpenAPITools/openapi-generator/pull/8198)

Dart:

• There are problems regarding deserialization of maps, de-serialization code is trying to call Map.mapFromJson. There are open issues for this and the problem is now evident in the generated fake petstore.
  Related issues: https://github.com/OpenAPITools/openapi-generator/issues/8029
• De-serialization of maps of maps e.g. Map<String, Map<String, String>>
• De-serialization of maps of enums e.g. Map<String, Enum>
• De-serialization of list of list of model e.g. List<List<ReadOnlyFirst>>
• Support enums of type double, de-serialization via switch statement doesn't work here
• De-serialization of List<Object> tries to call Object.mapFromJson
• OAS3 fake petstore contains a model called Client, this conflict with http.Client (addressed by https://github.com/OpenAPITools/openapi-generator/pull/8198)

Dart-Dio:

• Compile errors regarding header parameters of type bool and List, basically everything that is not a String (addressed by #8191)

Start with the generated OAS3 fake petstore and issue #8029, reproducing the remaining Dart deserialization cases listed here: nested maps, maps of enums, nested model lists, double enums, and List. Done means the generated Dart and Dart-Dio clients handle those cases without invalid deserialization calls or compile errors.