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>

plugin/plugin.go

@@ -20,9 +20,7 @@ import (
 	"fmt"
 	"sync"
 
-	"github.com/containerd/ttrpc"
 	"github.com/pkg/errors"
-	"google.golang.org/grpc"
 )
 
 var (
@@ -63,6 +61,8 @@ const (
 	ServicePlugin Type = "io.containerd.service.v1"
 	// GRPCPlugin implements a grpc service
 	GRPCPlugin Type = "io.containerd.grpc.v1"
+	// TTRPCPlugin implements a ttrpc shim service
+	TTRPCPlugin Type = "io.containerd.ttrpc.v1"
 	// SnapshotPlugin implements a snapshotter
 	SnapshotPlugin Type = "io.containerd.snapshotter.v1"
 	// TaskMonitorPlugin implements a task monitor
@@ -124,21 +124,6 @@ func (r *Registration) URI() string {
 	return fmt.Sprintf("%s.%s", r.Type, r.ID)
 }
 
-// Service allows GRPC services to be registered with the underlying server
-type Service interface {
-	Register(*grpc.Server) error
-}
-
-// TTRPCService allows TTRPC services to be registered with the underlying server
-type TTRPCService interface {
-	RegisterTTRPC(*ttrpc.Server) error
-}
-
-// TCPService allows GRPC services to be registered with the underlying tcp server
-type TCPService interface {
-	RegisterTCP(*grpc.Server) error
-}
-
 var register = struct {
 	sync.RWMutex
 	r []*Registration