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/portforward.go

@@ -0,0 +1,19 @@
+package env
+
+import (
+	"fmt"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+)
+
+func readPortForward() (
+	portForwarding settings.PortForwarding, err error) {
+	portForwarding.Enabled, err = envToBoolPtr("PORT_FORWARDING")
+	if err != nil {
+		return portForwarding, fmt.Errorf("environment variable PORT_FORWARDING: %w", err)
+	}
+
+	portForwarding.Filepath = envToStringPtr("PORT_FORWARDING_STATUS_FILE")
+
+	return portForwarding, nil
+}