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/provider/vyprvpn/connection.go

@@ -4,7 +4,7 @@ import (
 	"errors"
 	"fmt"
 
-	"github.com/qdm12/gluetun/internal/configuration"
+	"github.com/qdm12/gluetun/internal/configuration/settings"
 	"github.com/qdm12/gluetun/internal/constants"
 	"github.com/qdm12/gluetun/internal/models"
 	"github.com/qdm12/gluetun/internal/provider/utils"
@@ -12,11 +12,11 @@ import (
 
 var ErrProtocolUnsupported = errors.New("network protocol is not supported")
 
-func (v *Vyprvpn) GetConnection(selection configuration.ServerSelection) (
+func (v *Vyprvpn) GetConnection(selection settings.ServerSelection) (
 	connection models.Connection, err error) {
 	const port = 443
 	const protocol = constants.UDP
-	if selection.OpenVPN.TCP {
+	if *selection.OpenVPN.TCP {
 		return connection, fmt.Errorf("%w: TCP for provider VyprVPN", ErrProtocolUnsupported)
 	}