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/wireguard.go

@@ -0,0 +1,44 @@
+package env
+
+import (
+	"fmt"
+	"net"
+	"os"
+	"strings"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+)
+
+func readWireguard() (wireguard settings.Wireguard, err error) {
+	defer func() {
+		err = unsetEnvKeys([]string{"WIREGUARD_PRIVATE_KEY", "WIREGUARD_PRESHARED_KEY"}, err)
+	}()
+	wireguard.PrivateKey = envToStringPtr("WIREGUARD_PRIVATE_KEY")
+	wireguard.PreSharedKey = envToStringPtr("WIREGUARD_PRESHARED_KEY")
+	wireguard.Interface = os.Getenv("WIREGUARD_INTERFACE")
+	wireguard.Addresses, err = readWireguardAddresses()
+	if err != nil {
+		return wireguard, err // already wrapped
+	}
+	return wireguard, nil
+}
+
+func readWireguardAddresses() (addresses []net.IPNet, err error) {
+	addressesCSV := os.Getenv("WIREGUARD_ADDRESS")
+	if addressesCSV == "" {
+		return nil, nil
+	}
+
+	addressStrings := strings.Split(addressesCSV, ",")
+	addresses = make([]net.IPNet, len(addressStrings))
+	for i, addressString := range addressStrings {
+		ip, ipNet, err := net.ParseCIDR(addressString)
+		if err != nil {
+			return nil, fmt.Errorf("environment variable WIREGUARD_ADDRESS: %w", err)
+		}
+		ipNet.IP = ip
+		addresses[i] = *ipNet
+	}
+
+	return addresses, nil
+}