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/dns/ticker.go

@@ -18,8 +18,8 @@ func (l *Loop) RunRestartTicker(ctx context.Context, done chan<- struct{}) {
 	timer.Stop()
 	timerIsStopped := true
 	settings := l.GetSettings()
-	if settings.UpdatePeriod > 0 {
-		timer.Reset(settings.UpdatePeriod)
+	if period := *settings.DoT.UpdatePeriod; period > 0 {
+		timer.Reset(period)
 		timerIsStopped = false
 	}
 	lastTick := time.Unix(0, 0)
@@ -47,14 +47,14 @@ func (l *Loop) RunRestartTicker(ctx context.Context, done chan<- struct{}) {
 			_, _ = l.statusManager.ApplyStatus(ctx, constants.Running)
 
 			settings := l.GetSettings()
-			timer.Reset(settings.UpdatePeriod)
+			timer.Reset(*settings.DoT.UpdatePeriod)
 		case <-l.updateTicker:
 			if !timer.Stop() {
 				<-timer.C
 			}
 			timerIsStopped = true
 			settings := l.GetSettings()
-			newUpdatePeriod := settings.UpdatePeriod
+			newUpdatePeriod := *settings.DoT.UpdatePeriod
 			if newUpdatePeriod == 0 {
 				continue
 			}