Add sandbox store plugin type

Moves the sandbox store plugin under the plugins packages and adds a
unique plugin type for other plugins to depend on it.
Updates the sandbox controller plugin to depend on the sandbox store
plugin.

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

plugins/sandbox/store.go

@@ -0,0 +1,40 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package sandbox
+
+import (
+	"github.com/containerd/containerd/metadata"
+	"github.com/containerd/containerd/plugin"
+)
+
+func init() {
+	plugin.Register(&plugin.Registration{
+		Type: plugin.SandboxStorePlugin,
+		ID:   "local",
+		Requires: []plugin.Type{
+			plugin.MetadataPlugin,
+		},
+		InitFn: func(ic *plugin.InitContext) (interface{}, error) {
+			m, err := ic.Get(plugin.MetadataPlugin)
+			if err != nil {
+				return nil, err
+			}
+
+			return metadata.NewSandboxStore(m.(*metadata.DB)), nil
+		},
+	})
+}