Implement fs.FS for backends and add helper

[wojas]

To allow streaming reading of larger blobs, implement the fs.FS interface in all backends.

For convenience, also provide a fallback implementation that simply uses the existing Interface to load the data.

To retrieve the FS interface for a backend, a program would call this convenience method:

// AsFS returns an fs.FS implementation for the given storage backend.
// If the storage backend implements fs.FS, that is returned. If it does not, a generic implementation
// that implements it using Interface is returned instead.
func AsFS(st Interface) fs.FS