Introduce Walk Method Per Storage Driver

Move the Walk types into registry/storage/driver, and add a Walk method to each
storage driver. Although this is yet another API to implement, there is a fall
back implementation that relies on List and Stat. For some filesystems this is
very slow.

Also, this WalkDir Method conforms better do a traditional WalkDir (a la filepath).

This change is in preparation for refactoring.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>

registry/storage/driver/storagedriver.go

@@ -83,6 +83,13 @@ type StorageDriver interface {
 	// May return an ErrUnsupportedMethod in certain StorageDriver
 	// implementations.
 	URLFor(ctx context.Context, path string, options map[string]interface{}) (string, error)
+
+	// Walk traverses a filesystem defined within driver, starting
+	// from the given path, calling f on each file.
+	// If the returned error from the WalkFn is ErrSkipDir and fileInfo refers
+	// to a directory, the directory will not be entered and Walk
+	// will continue the traversal.  If fileInfo refers to a normal file, processing stops
+	Walk(ctx context.Context, path string, f WalkFn) error
 }
 
 // FileWriter provides an abstraction for an opened writable file-like object in