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/publicip/loop.go

@@ -4,7 +4,7 @@ import (
 	"net/http"
 	"time"
 
-	"github.com/qdm12/gluetun/internal/configuration"
+	"github.com/qdm12/gluetun/internal/configuration/settings"
 	"github.com/qdm12/gluetun/internal/constants"
 	"github.com/qdm12/gluetun/internal/loopstate"
 	"github.com/qdm12/gluetun/internal/models"
@@ -47,7 +47,7 @@ type Loop struct {
 const defaultBackoffTime = 5 * time.Second
 
 func NewLoop(client *http.Client, logger Logger,
-	settings configuration.PublicIP, puid, pgid int) *Loop {
+	settings settings.PublicIP, puid, pgid int) *Loop {
 	start := make(chan struct{})
 	running := make(chan models.LoopStatus)
 	stop := make(chan struct{})