nri: add experimental NRI plugin.

Add a common NRI 'service' plugin. It takes care of relaying
requests and respones to and from NRI (external NRI plugins)
and the high-level containerd namespace-independent logic of
applying NRI container adjustments and updates to actual CRI
and other containers.

The namespace-dependent details of the necessary container
manipulation operations are to be implemented by namespace-
specific adaptations. This NRI plugin defines the API which
such adaptations need to implement.

Signed-off-by: Krisztian Litkey <krisztian.litkey@intel.com>

pkg/nri/plugin/plugin.go

@@ -0,0 +1,36 @@
+/*
+   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 plugin
+
+import (
+	"github.com/containerd/containerd/pkg/nri"
+	"github.com/containerd/containerd/plugin"
+)
+
+func init() {
+	plugin.Register(&plugin.Registration{
+		Type:   plugin.NRIApiPlugin,
+		ID:     "nri",
+		Config: nri.DefaultConfig(),
+		InitFn: initFunc,
+	})
+}
+
+func initFunc(ic *plugin.InitContext) (interface{}, error) {
+	l, err := nri.New(ic.Config.(*nri.Config))
+	return l, err
+}