Define and set a unique user-agent

[goeffthomas]

If the metadata for Croissant is pulled via URL (done here), we should set a user-agent that allows the package to be identified.

For reference, kagglehub does something similar here

[goeffthomas]

We should probably add that same user agent when downloading the files over HTTP: https://github.com/mlcommons/croissant/blob/main/python/mlcroissant/mlcroissant/_src/operation_graph/operations/download.py#L188-L190