Add field to Container for client-defined data

This field allows a client to store specialized information in the
container metadata rather than having to store this itself and keep
the data in sync with containerd.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

containerstore.go

@@ -97,6 +97,7 @@ func containerToProto(container *containers.Container) containersapi.Container {
 		Spec:        container.Spec,
 		Snapshotter: container.Snapshotter,
 		SnapshotKey: container.SnapshotKey,
+		Extensions:  container.Extensions,
 	}
 }
 
@@ -116,6 +117,7 @@ func containerFromProto(containerpb *containersapi.Container) containers.Contain
 		Spec:        containerpb.Spec,
 		Snapshotter: containerpb.Snapshotter,
 		SnapshotKey: containerpb.SnapshotKey,
+		Extensions:  containerpb.Extensions,
 	}
 }