Add cool-off functionality

[pelson]

pypa/pip#13674

https://github.com/simple-repository/simple-repository-server/pull/27/files#r2675348692

https://blog.yossarian.net/2025/11/21/We-should-all-be-using-dependency-cooldowns

It would be good to make it easy to run a simple repo server with cool-off functionality. I'm hesitant to add a global parameter to the simple-repository-server CLI, as it might require some nuance (like whitelisting certain packages).