Improve output formatting for s3torch-benchmark

[muddyfish]

Tell us more about this new feature.

Right now, the s3torch-benchmark script outputs results to stdout in a bespoke logging format. Visualising the data, or otherwise copying it to a tabular format can only be done manually, or writing another script to parse the data.

I'd recommend supporting CSV/JSON file output, with options to specify the dimensions for rows/columns, and the rest allowable to be templated in a filename. Outputting to an image plot would be great as well