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/custom/openvpnconf_test.go

@@ -4,18 +4,23 @@ import (
 	"net"
 	"testing"
 
-	"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/stretchr/testify/assert"
 )
 
+func boolPtr(b bool) *bool       { return &b }
+func intPtr(n int) *int          { return &n }
+func uint16Ptr(n uint16) *uint16 { return &n }
+func stringPtr(s string) *string { return &s }
+
 func Test_modifyConfig(t *testing.T) {
 	t.Parallel()
 
 	testCases := map[string]struct {
 		lines      []string
-		settings   configuration.OpenVPN
+		settings   settings.OpenVPN
 		connection models.Connection
 		modified   []string
 	}{
@@ -30,14 +35,16 @@ func Test_modifyConfig(t *testing.T) {
 				"keep me here",
 				"auth bla",
 			},
-			settings: configuration.OpenVPN{
+			settings: settings.OpenVPN{
 				User:      "user",
 				Ciphers:   []string{"cipher"},
-				Auth:      "auth",
-				MSSFix:    1000,
+				Auth:      stringPtr("auth"),
+				MSSFix:    uint16Ptr(1000),
+				Root:      boolPtr(false),
 				ProcUser:  "procuser",
 				Interface: "tun3",
-			},
+				Verbosity: intPtr(0),
+			}.WithDefaults(constants.Custom),
 			connection: models.Connection{
 				IP:       net.IPv4(1, 2, 3, 4),
 				Port:     1194,