chore(settings): refactor settings processing (#756)

- Better settings tree structure logged using `qdm12/gotree`
- Read settings from environment variables, then files, then secret files
- Settings methods to default them, merge them and override them
- `DNS_PLAINTEXT_ADDRESS` default changed to `127.0.0.1` to use DoT. Warning added if set to something else.
- `HTTPPROXY_LISTENING_ADDRESS` instead of `HTTPPROXY_PORT` (with retro-compatibility)

internal/wireguard/settings.go

@@ -94,7 +94,7 @@ func (s *Settings) Check() (err error) {
 	switch {
 	case s.Endpoint == nil:
 		return ErrEndpointMissing
-	case s.Endpoint.IP == nil:
+	case len(s.Endpoint.IP) == 0:
 		return ErrEndpointIPMissing
 	case s.Endpoint.Port == 0:
 		return ErrEndpointPortMissing