Set up the upstream icons server

In some cases vaultwarden is deployed in a network environment, this network environment may be limited by firewall reasons, such as unable to access services such as google.com, so that Icon cannot be downloaded, I think vaultwarden if you can add an upstream icons service settings, This service is responsible for downloading Icon. And vaultwarden is just a reverse proxy, or download the icon based on this

For example, icons.bitwarden.net is an official address.