Add a settings backup & restore mechanism

I think it would be very convenient, especially for users who manually select resolvers and routes and that setup Simple DnsCrypt on multiple devices.

No files, tests, or entry points are named. Start by locating where manually selected resolvers and routes are stored, then determine the settings that must be portable across devices; done should mean a defined backup and restore flow, but the issue does not specify its format or scope.

Written by the indexing model from the issue text.