storage-operator: add package file to pkg/apis/storage

Without such file in place, code-generation using `operator-sdk generate
openapi` tends to fail. This is fixed in later versions of
`operator-sdk`, which generate exactly this file when scaffolding a new
API.

See: operator-framework/operator-sdk#1401
See: operator-framework/operator-sdk#1502
See: operator-framework/operator-sdk#1546

storage-operator/pkg/apis/storage/group.go

@@ -0,0 +1,6 @@
+// Package storage contains storage API versions.
+//
+// This file ensures Go source parsers acknowledge the storage package
+// and any child packages. It can be removed if any other Go source files are
+// added to this package.
+package storage