s3 URL in bfoptions

[will-moore]

See ome/ZarrReader#82

To take advantage of direct access to s3 via URL, we need to add that option to bfoptions that we're creating with mkngff.
Currently we're generating the same bfoptions file for every file.

Also it would be handy to update the bfoptions file for existing data.
Maybe a script that does the update could be used in production, instead of including URL in bfoptions when it's generated.