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/httpproxy/settings.go

@@ -3,17 +3,17 @@ package httpproxy
 import (
 	"context"
 
-	"github.com/qdm12/gluetun/internal/configuration"
+	"github.com/qdm12/gluetun/internal/configuration/settings"
 	"github.com/qdm12/gluetun/internal/httpproxy/state"
 )
 
 type SettingsGetSetter = state.SettingsGetSetter
 
-func (l *Loop) GetSettings() (settings configuration.HTTPProxy) {
+func (l *Loop) GetSettings() (settings settings.HTTPProxy) {
 	return l.state.GetSettings()
 }
 
-func (l *Loop) SetSettings(ctx context.Context, settings configuration.HTTPProxy) (
+func (l *Loop) SetSettings(ctx context.Context, settings settings.HTTPProxy) (
 	outcome string) {
 	return l.state.SetSettings(ctx, settings)
 }