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/vpn/run.go

@@ -29,9 +29,9 @@ func (l *Loop) Run(ctx context.Context, done chan<- struct{}) {
 	for ctx.Err() == nil {
 		settings, allServers := l.state.GetSettingsAndServers()
 
-		providerConf := provider.New(settings.Provider.Name, allServers, time.Now)
+		providerConf := provider.New(*settings.Provider.Name, allServers, time.Now)
 
-		portForwarding := settings.Provider.PortForwarding.Enabled
+		portForwarding := *settings.Provider.PortForwarding.Enabled
 		var vpnRunner vpnRunner
 		var serverName, vpnInterface string
 		var err error