client-go: factor the dynamic client similarly to others
client-go: factor the dynamic client similarly to others

All other clients:

 - expose a New() method that takes a rest.Interface
 - expose their RESTClient()
 - return pointers to the type, not instances of an interface that the
   type implements

For code that is generic over all Kubernetes clients, and for general
developer experience, it's best to make sure that this client adheres to
these common practices.

Signed-off-by: Steve Kuznetsov <skuznets@redhat.com>

Kubernetes-commit: 74af6f1e7d9057bfcb64f4d875063c14041937e5

kubernetes/client-go