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/secrets/httpproxy.go

@@ -0,0 +1,27 @@
+package secrets
+
+import (
+	"fmt"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+)
+
+func readHTTPProxy() (settings settings.HTTPProxy, err error) {
+	settings.User, err = readSecretFileAsStringPtr(
+		"HTTPPROXY_USER_SECRETFILE",
+		"/run/secrets/httpproxy_user",
+	)
+	if err != nil {
+		return settings, fmt.Errorf("cannot read HTTP proxy user secret file: %w", err)
+	}
+
+	settings.Password, err = readSecretFileAsStringPtr(
+		"HTTPPROXY_PASSWORD_SECRETFILE",
+		"/run/secrets/httpproxy_password",
+	)
+	if err != nil {
+		return settings, fmt.Errorf("cannot read OpenVPN password secret file: %w", err)
+	}
+
+	return settings, nil
+}