Add shared content label to namespaces

Adds shared content labels to namespaces allowing content to be shared
between namespaces if that namespace is specifically tagged as being
sharable by adding the `containerd.io/namespace/sharable` label to the
namespace.

Signed-off-by: Cody Roseborough <cdr@amazon.com>

labels/labels.go

@@ -19,3 +19,7 @@ package labels
 // LabelUncompressed is added to compressed layer contents.
 // The value is digest of the uncompressed content.
 const LabelUncompressed = "containerd.io/uncompressed"
+
+// LabelSharedNamespace is added to a namespace to allow that namespaces
+// contents to be shared.
+const LabelSharedNamespace = "containerd.io/namespace.shareable"