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/updater.go

@@ -0,0 +1,50 @@
+package env
+
+import (
+	"fmt"
+	"net"
+	"os"
+	"time"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+	"github.com/qdm12/gluetun/internal/constants"
+)
+
+func readUpdater() (updater settings.Updater, err error) {
+	updater.Period, err = readUpdaterPeriod()
+	if err != nil {
+		return updater, err
+	}
+
+	updater.DNSAddress, err = readUpdaterDNSAddress()
+	if err != nil {
+		return updater, err
+	}
+
+	// TODO use current provider being used
+	updater.Providers = constants.AllProviders()
+
+	return updater, nil
+}
+
+func readUpdaterPeriod() (period *time.Duration, err error) {
+	s := os.Getenv("UPDATER_PERIOD")
+	if s == "" {
+		return nil, nil //nolint:nilnil
+	}
+	period = new(time.Duration)
+	*period, err = time.ParseDuration(s)
+	if err != nil {
+		return nil, fmt.Errorf("environment variable UPDATER_PERIOD: %w", err)
+	}
+	return period, nil
+}
+
+func readUpdaterDNSAddress() (ip net.IP, err error) {
+	// TODO this is currently using Cloudflare in
+	// plaintext to not be blocked by DNS over TLS by default.
+	// If a plaintext address is set in the DNS settings, this one will be used.
+	// use custom future encrypted DNS written in Go without blocking
+	// as it's too much trouble to start another parallel unbound instance for now.
+	return nil, nil
+}