Add AmazonS3 and Http storage backends

[zoj613]

We could add a storage backend for reading and writing from a Zarr hierarchy stored in an Amazon S3 bucket. This should be useful since a lot of public Zarr datasets are hosted on Amazon S3.

We could make use of https://github.com/andersfugmann/aws-s3 to implement this AmazonS3Store. It also supports Lwt for concurrent reads/writes.

For an http store, we could use https://github.com/mirage/ocaml-cohttp