Add support for shim plugins

Refactor shim v2 to load and register plugins.
Update init shim interface to not require task service implementation on
returned service, but register as plugin if it is.

Signed-off-by: Derek McGowan <derek@mcg.dev>

pkg/cri/server/service.go

@@ -65,7 +65,7 @@ type CRIService interface {
 	Run() error
 	// io.Closer is used by containerd to gracefully stop cri service.
 	io.Closer
-	plugin.Service
+	Register(*grpc.Server) error
 	grpcServices
 }