Add Load for container and Task with Attach

This adds both container and task loading of running tasks as well as
reattaching to the IO of the task after load.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

client.go

@@ -203,6 +203,16 @@ func (c *Client) NewContainer(ctx context.Context, id string, spec *specs.Spec,
 	return containerFromProto(c, r.Container), nil
 }
 
+func (c *Client) LoadContainer(ctx context.Context, id string) (Container, error) {
+	response, err := c.ContainerService().Get(ctx, &containers.GetContainerRequest{
+		ID: id,
+	})
+	if err != nil {
+		return nil, err
+	}
+	return containerFromProto(c, response.Container), nil
+}
+
 type RemoteOpts func(*Client, *RemoteContext) error
 
 // RemoteContext is used to configure object resolutions and transfers with