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/version.go

@@ -0,0 +1,33 @@
+package env
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+	"github.com/qdm12/govalid/binary"
+)
+
+func readVersion() (version settings.Version, err error) {
+	version.Enabled, err = readVersionEnabled()
+	if err != nil {
+		return version, err
+	}
+
+	return version, nil
+}
+
+func readVersionEnabled() (enabled *bool, err error) {
+	s := os.Getenv("VERSION_INFORMATION")
+	if s == "" {
+		return nil, nil //nolint:nilnil
+	}
+
+	enabled = new(bool)
+	*enabled, err = binary.Validate(s)
+	if err != nil {
+		return nil, fmt.Errorf("environment variable VERSION_INFORMATION: %w", err)
+	}
+
+	return enabled, nil
+}