Consider retrying URLs in descriptor

[danlamanna]

For bigger zip files it would be nice to support retrying downloads in the event of intermittent upstream errors.

[scosman]

Not an immediate priority, but if anyone with this this issue in the wild want to contribute this I'd take it in main. Would need to be a config option (ZS_MAX_UPSTREAM_RETRIES), with default still zero retries for backwards compatibility (and I think that's the right default).