use fsspec as ObjectStore backend

[kylebarron]

Of the required methods of the ObjectStore trait, they mostly seem pretty straightforward. I'd probably start with implementing delete, list, get_opts, and copy, and leave put_opts and especially put_multipart_opts for future work.

put_opts

Should be straightforward with put_file.

put_multipart_opts

Not sure if this one is really possible? At any rate, best to leave as todo!() and come back to later.

get_opts

This should be pretty easy to implement with cat_ranges

delete

Easy with _rm.

list

Should be possible with _ls.

Would need to ensure we can create an ObjectMeta out of the response.

• Can we assume the response has a timestamp?

list_with_delimiter

Would need to create common_prefixes in the ListResult.

copy

Should be easy with copy

copy_if_not_exists

Not sure if fsspec has this natively, but we can fake it (subject to race conditions) by first checking exists.

get_ranges

ObjectStore::get_ranges is not a required trait method, but we could implement it if we want the coalescing to happen on the fsspec side rather than the object-store side.

cc @martindurant

[kylebarron]

This should be moved to https://github.com/developmentseed/object-store-rs