Snapshotters: Export the root path

Some of the snapshotters that allow you to change their root location
were already doing this, this just makes all of them follow the same
pattern.

Signed-off-by: Danny Canter <danny@dcantah.dev>

plugins/types.go

@@ -93,3 +93,7 @@ const (
 	// PropertyTTRPCAddress is the ttrpc address used for client connections to containerd
 	PropertyTTRPCAddress = "io.containerd.plugin.ttrpc.address"
 )
+
+const (
+	SnapshotterRootDir = "root"
+)