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/configuration/sources/env/wireguardselection.go

@@ -0,0 +1,65 @@
+package env
+
+import (
+	"errors"
+	"fmt"
+	"net"
+	"os"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+	"github.com/qdm12/govalid/port"
+)
+
+func (r *Reader) readWireguardSelection() (
+	selection settings.WireguardSelection, err error) {
+	selection.EndpointIP, err = readWireguardEndpointIP()
+	if err != nil {
+		return selection, err
+	}
+
+	selection.EndpointPort, err = r.readWireguardCustomPort()
+	if err != nil {
+		return selection, err
+	}
+
+	selection.PublicKey = os.Getenv("WIREGUARD_PUBLIC_KEY")
+
+	return selection, nil
+}
+
+var ErrIPAddressParse = errors.New("cannot parse IP address")
+
+func readWireguardEndpointIP() (endpointIP net.IP, err error) {
+	s := os.Getenv("WIREGUARD_ENDPOINT_IP")
+	if s == "" {
+		return nil, nil
+	}
+	endpointIP = net.ParseIP(s)
+	if endpointIP == nil {
+		return nil, fmt.Errorf("environment variable WIREGUARD_ENDPOINT_IP: %w: %s",
+			ErrIPAddressParse, s)
+	}
+	return endpointIP, nil
+}
+
+func (r *Reader) readWireguardCustomPort() (customPort *uint16, err error) {
+	key := "WIREGUARD_ENDPOINT_PORT"
+	s := os.Getenv(key)
+	if s == "" {
+		// Retro-compatibility
+		key = "WIREGUARD_PORT"
+		s = os.Getenv(key)
+		if s == "" {
+			return nil, nil //nolint:nilnil
+		}
+		r.onRetroActive("WIREGUARD_PORT", "WIREGUARD_ENDPOINT_PORT")
+	}
+
+	customPort = new(uint16)
+	*customPort, err = port.Validate(s)
+	if err != nil {
+		return nil, fmt.Errorf("environment variable %s: %w", key, err)
+	}
+
+	return customPort, nil
+}