We need a solution for API Explorer + Custom ModelBinder

see: https://github.com/aspnet/Mvc/issues/5673#issuecomment-281738074 and https://github.com/domaindrivendev/Swashbuckle.AspNetCore/issues/309

The complaint here is that we 'expand' a parameter in the API definition that's being bound by a custom model binder. API explorer has no interaction with model binders/providers, and so no way to know that this isn't being handled by the default binder.

In this case we already have a metadata feature that would give the desired API explorer output, `ModelMetadata.IsComplexType == false`. However we don't allow user code to configure this value.

We should either make this configurable, or probe the model binders for information about types that have custom handling.