Include storage options in references?

[martindurant]

Currently, we include only the URLs of references in the generated JSON files. With fsspec/filesystem_spec#867 , this becomes more complicated, as we can have multiple remote filesystems, each with their own set of optional storage_options.

Since the notebook cells instantiating a ReferenceFileSystem were already getting rather large (although you can encode these in an Intake entry), perhaps we could encode these options directly into the references? On the downside, this makes is less convenient to relocate the original data, by providing an override for URL templates - but I don't know if that feature was being used in practice.