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/azure/azure.go

@@ -336,6 +336,12 @@ func (d *driver) URLFor(ctx context.Context, path string, options map[string]int
 	return d.client.GetBlobSASURI(d.container, path, expiresTime, "r")
 }
 
+// Walk traverses a filesystem defined within driver, starting
+// from the given path, calling f on each file
+func (d *driver) Walk(ctx context.Context, path string, f storagedriver.WalkFn) error {
+	return storagedriver.WalkFallback(ctx, d, path, f)
+}
+
 // directDescendants will find direct descendants (blobs or virtual containers)
 // of from list of blob paths and will return their full paths. Elements in blobs
 // list must be prefixed with a "/" and