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/health.go

@@ -0,0 +1,52 @@
+package env
+
+import (
+	"fmt"
+	"os"
+	"time"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+)
+
+func (r *Reader) ReadHealth() (health settings.Health, err error) {
+	health.ServerAddress = os.Getenv("HEALTH_SERVER_ADDRESS")
+	health.AddressToPing = os.Getenv("HEALTH_ADDRESS_TO_PING")
+
+	health.VPN.Initial, err = r.readDurationWithRetro(
+		"HEALTH_VPN_DURATION_INITIAL",
+		"HEALTH_OPENVPN_DURATION_INITIAL")
+	if err != nil {
+		return health, err
+	}
+
+	health.VPN.Initial, err = r.readDurationWithRetro(
+		"HEALTH_VPN_DURATION_ADDITION",
+		"HEALTH_OPENVPN_DURATION_ADDITION")
+	if err != nil {
+		return health, err
+	}
+
+	return health, nil
+}
+
+func (r *Reader) readDurationWithRetro(envKey, retroEnvKey string) (d *time.Duration, err error) {
+	s := os.Getenv(envKey)
+	if s == "" {
+		s = os.Getenv(retroEnvKey)
+		if s == "" {
+			return nil, nil //nolint:nilnil
+		}
+		r.onRetroActive(envKey, retroEnvKey)
+		envKey = retroEnvKey
+	}
+
+	d = new(time.Duration)
+	*d, err = time.ParseDuration(s)
+	if err != nil {
+		return nil, fmt.Errorf(
+			"environment variable %s: %w",
+			envKey, err)
+	}
+
+	return d, nil
+}