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>

containers/containers.go

@@ -57,6 +57,9 @@ type Container struct {
 
 	// UpdatedAt is the time at which the container was updated.
 	UpdatedAt time.Time
+
+	// Extensions stores client-specified metadata
+	Extensions []types.Any
 }
 
 type RuntimeInfo struct {