We need a solution for API Explorer + Custom ModelBinder

[rynowak]

see: aspnet/Mvc#5673 (comment) and domaindrivendev/Swashbuckle.AspNetCore#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.

[rynowak]

/cc @domaindrivendev

[pranavkm]

Moving this to 3.0