exec credential provider: ProvideClusterInfo and kubeconfig shadow

- The main idea here is that we want to 1) prevent potentially large CA
  bundles from being set in an exec plugin's environment and 2) ensure
  that the exec plugin is getting everything it needs in order to talk to
  a cluster.
- Avoid breaking existing manual declarations of rest.Config instances by
  moving exec Cluster to kubeconfig internal type.
- Use client.authentication.k8s.io/exec to qualify exec cluster extension.
- Deep copy the exec Cluster.Config when we copy a rest.Config.

Signed-off-by: Andrew Keesler <akeesler@vmware.com>

Kubernetes-commit: c4299d15d5289768808034676858e76a177eeae5

tools/clientcmd/api/zz_generated.deepcopy.go

@@ -267,6 +267,9 @@ func (in *ExecConfig) DeepCopyInto(out *ExecConfig) {
 		*out = make([]ExecEnvVar, len(*in))
 		copy(*out, *in)
 	}
+	if in.Config != nil {
+		out.Config = in.Config.DeepCopyObject()
+	}
 	return
 }