If we have a FromResponseUnmarshaller for T, then we can turn a asClient from AsClient[A, HttpResponse] to AsClient[A, T]
If we have a FromResponseUnmarshaller for T, then we can turn a asClient from AsClient[A, HttpResponse] to AsClient[A, T]
the unmarshalled type
a materializer for unmarshalling
a asClient from AsClient[A, HttpResponse] to AsClient[A, T]