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/vpn/openvpn.go

@@ -5,7 +5,7 @@ import (
 	"errors"
 	"fmt"
 
-	"github.com/qdm12/gluetun/internal/configuration"
+	"github.com/qdm12/gluetun/internal/configuration/settings"
 	"github.com/qdm12/gluetun/internal/firewall"
 	"github.com/qdm12/gluetun/internal/openvpn"
 	"github.com/qdm12/gluetun/internal/provider"
@@ -24,7 +24,7 @@ var (
 // It returns a serverName for port forwarding (PIA) and an error if it fails.
 func setupOpenVPN(ctx context.Context, fw firewall.VPNConnectionSetter,
 	openvpnConf openvpn.Interface, providerConf provider.Provider,
-	settings configuration.VPN, starter command.Starter, logger openvpn.Logger) (
+	settings settings.VPN, starter command.Starter, logger openvpn.Logger) (
 	runner vpnRunner, serverName string, err error) {
 	connection, err := providerConf.GetConnection(settings.Provider.ServerSelection)
 	if err != nil {