refactor: Improve service return values + failure definition

[jtdLab]

Instead of Either<FooFailure, FooValue> + a freezed FooFailure Union consider using a sealed class FooResult having success and failure subclasses.

[jtdLab]

Goal is writing something like:

abstract class IMyService {
    MyServiceMyMethodResult myMethod();
    ... more methods
}

sealed class MyServiceMyMethodResult {
    data class MyServiceMyMethodSuccess(Object returnValue);
    sealed class MyServiceMyMethodFailure {
        data class MyServiceMyMethodFailureA({String? someProp);
        data class MyServiceMyMethodFailureB({required int someOtherProp});
        final class MyServiceMyMethodFailureC();
        ... more failure cases
    }
}

... more method results

// Failures that are shared across multiple methods (This is only an idea -> sharing failures might be bit overkill)
final class MyServiceMySharedFailureA implements MyServiceMyMethodFailure /* more method failure impls go here to get exhaustive switch */{}
... more shared failures

This needs data classes and nested sealed classes to be added to the dart language to be an improvement. Currently freezed does not support nesting unions so we stick with Either and freezed sealed & union classes for now