Add log rotation for custom script extension logs

We've encountered an issue where our custom script extension generates very large logging output (100+ MB). As these files don't seem to be cleaned up frequently (or at all?), the logs accumulate in /var/lib/waagent/custom-script/downloads and end up using huge amounts of disk space.

Can log rotation for these files be configured?

Start by locating the code that manages files under /var/lib/waagent/custom-script/downloads and any existing cleanup or logging behavior. Determine how log rotation could be configured for accumulated custom script logs, then verify that large log files are rotated or removed without affecting script execution.

Written by the indexing model from the issue text.