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?

首先定位管理 /var/lib/waagent/custom-script/downloads 下文件的代码，以及任何现有的清理或日志记录行为。确定如何为累积的 custom script 日志配置日志轮换，然后验证大型日志文件是否会在不影响脚本执行的情况下进行轮换或删除。

由索引模型根据 Issue 内容生成。