[FEATURE] Add abstract generic support

[Nitero]

Feature description

First of all thanks for the great package, it works like a charm!

So I'm not sure if this is possible, but it would be nice if we could use SubclassSelector on abstract classes with a generic type. This way we could add an interface to the abstract class that does not have a generic type. Then we could use this to specify config values in a dictionary without a type.

My workaround for this is to use a generic interface and have a second plain interface just for selecting targets easily in configs.

public class Example : MonoBehaviour
{
    [SerializeReference, SubclassSelector]
    private Target<Character> _target;// does NOT show ConcreteTarget

    [SerializeReference, SubclassSelector]
    private ITarget<Character> _target2;// DOES show ConcreteTarget

    // [SerializeReference, SubclassSelector]
    // private SerializedDictionary<ITarget, float> _configs;// not possible as the interface needs a generic type
    
    [SerializeReference, SubclassSelector]
    private SerializedDictionary<ITargetConfigurable, float> _configs2;// possible workaround
}


// ideally we could add the generic type in the abstract class instead
public interface ITarget<T>
{
    T GetTargets();
}

// using an extra interface instead of ITarget allows us to select a target without type eg. in a config
public interface ITargetConfigurable { }

public abstract class Target<T> : ITarget<T>, ITargetConfigurable
{
    public abstract T GetTargets();
}

[Serializable]
public class ConcreteTarget : Target<Character>
{
    public override Character GetTargets() => null;
}

public class Character { }