Add a Power User Preferences option `external_host` to publish DDNS hostnames for server discovery

[gladandong]

I live in a country where Resilio Sync trackers have been blocked. It is not possible for my NAS connect to the trackers without going through some sort of VPN. One downside of this approach is the tracker has no way to know my own Internet IP address, it will take the IP address of the VPN server instead.

There is an Power User Preferences option named `external_port`, I am wondering if it possible to add a new one named `external_host`, whose value can be a single or list of comma seperated DDNS names, and update the tracker to take value of this option instead of source IP of the request.

Thanks in advance!