[sandbox] Implement sandbox controller

Signed-off-by: Maksym Pavlenko <pavlenko.maksym@gmail.com>

runtime/v2/shim.go

@@ -194,6 +194,8 @@ type ShimProcess interface {
 	ID() string
 	// Namespace of this shim.
 	Namespace() string
+	// Client returns the underlying TTRPC client for this shim.
+	Client() *ttrpc.Client
 }
 
 type shim struct {
@@ -243,6 +245,10 @@ type shimTask struct {
 	task task.TaskService
 }
 
+func (s *shimTask) Client() *ttrpc.Client {
+	return s.client
+}
+
 func (s *shimTask) Shutdown(ctx context.Context) error {
 	_, err := s.task.Shutdown(ctx, &task.ShutdownRequest{
 		ID: s.ID(),