Add route to fetch closest ISO URL

[jonathanspw]

Something similar to /mirrorlist but that will only return the base URL of the closest mirror. Perhaps we could simply add a route for /mirrorlist/<version> to serve this list of base URLs.

This would allow a quality of life improvement in https://github.com/AlmaLinux/cloud-images so that it can fetch ISOs from a better mirror than the currently hard-coded repo.almalinux.org.

This can probably be used in other automated processes and projects as well.