Support getting full / relative paths from storage.Object

[rdjjke]

Files in storages now are represented by storage.Object interface and its storage.LocalObject implementation.

They're usually used in two different scenarious:

1. Storage implementations provide LocalObjects from ListFolder funcs and in this case objects contain file names in their GetName.
2. New LocalObjects are created in the other code, for example in storage.ListFolderRecursively*** / multistorage.ListFolderRecursively*** funcs. Such objects may be constructed in a different way, for example, GetName may contain full / relative paths instead of just names.

The problem is that it's not always clear from the code what kind of object in hidden by the storage.Object interface: the original one provided from the storage, or a derivative one, which may contain a path in its GetName.

We should introduce a way to distinguish somehow between these different kinds of objects. The other option, and may be the better one, is to add the new GetPath() func to the storage.Object interface so that we won't need to create new derivative structs implementing storage.Object anywhere outside storage implementations.