Add s3 stats script

[dominikl]

Add script to process s3 summary stats (for idr release stats).

Will print out:

• Total number of bytes
• Total number of zarrs
• Average size of zarr

[sbesson]

aws s3 ls --summarize would also be my first pick for retrieving all object metadata under a prefix on S3, which is amenable to a zarr. There are a few --output options including tabular/YAML format which might facilitate part of the logic of this script. For the tabular output, it might be possible to use tooling like pandas.

It would certainly be useful to have some additional input from the IDR team on the new requirements regarding the stats collection and reporting in the new era where all/part of the data will be available via object stores and/or archives.