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>

plugin/plugin.go

@@ -80,6 +80,8 @@ const (
 	LeasePlugin Type = "io.containerd.lease.v1"
 	// TracingProcessorPlugin implements a open telemetry span processor
 	TracingProcessorPlugin Type = "io.containerd.tracing.processor.v1"
+	// NRIApiPlugin implements the NRI adaptation interface for containerd.
+	NRIApiPlugin Type = "io.containerd.nri.v1"
 )
 
 const (