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/configuration/settings/surfshark_retro.go

@@ -0,0 +1,56 @@
+package settings
+
+import (
+	"strings"
+
+	"github.com/qdm12/gluetun/internal/constants"
+	"github.com/qdm12/gluetun/internal/models"
+)
+
+func surfsharkRetroRegion(selection ServerSelection) (
+	updatedSelection ServerSelection) {
+	locationData := constants.SurfsharkLocationData()
+
+	retroToLocation := make(map[string]models.SurfsharkLocationData, len(locationData))
+	for _, data := range locationData {
+		if data.RetroLoc == "" {
+			continue
+		}
+		retroToLocation[strings.ToLower(data.RetroLoc)] = data
+	}
+
+	for i, region := range selection.Regions {
+		location, ok := retroToLocation[region]
+		if !ok {
+			continue
+		}
+		selection.Regions[i] = strings.ToLower(location.Region)
+		selection.Countries = append(selection.Countries, strings.ToLower(location.Country))
+		selection.Cities = append(selection.Cities, strings.ToLower(location.City)) // even empty string
+		selection.Hostnames = append(selection.Hostnames, location.Hostname)
+	}
+
+	selection.Regions = dedupSlice(selection.Regions)
+	selection.Countries = dedupSlice(selection.Countries)
+	selection.Cities = dedupSlice(selection.Cities)
+	selection.Hostnames = dedupSlice(selection.Hostnames)
+
+	return selection
+}
+
+func dedupSlice(slice []string) (deduped []string) {
+	if slice == nil {
+		return nil
+	}
+
+	deduped = make([]string, 0, len(slice))
+	seen := make(map[string]struct{}, len(slice))
+	for _, s := range slice {
+		if _, ok := seen[s]; !ok {
+			seen[s] = struct{}{}
+			deduped = append(deduped, s)
+		}
+	}
+
+	return deduped
+}