feat: add VPNsecure.me support (#848)

- `OPENVPN_ENCRYPTED_KEY` environment variable 
- `OPENVPN_ENCRYPTED_KEY_SECRETFILE` environment variable 
- `OPENVPN_KEY_PASSPHRASE` environment variable 
- `OPENVPN_KEY_PASSPHRASE_SECRETFILE` environment variable 
- `PREMIUM_ONLY` environment variable
- OpenVPN user and password not required for vpnsecure provider

internal/storage/formatting.go

@@ -114,6 +114,10 @@ func noServerFoundError(selection settings.ServerSelection) (err error) {
 		messageParts = append(messageParts, "free tier only")
 	}
 
+	if *selection.PremiumOnly {
+		messageParts = append(messageParts, "premium tier only")
+	}
+
 	message := "for " + strings.Join(messageParts, "; ")
 
 	return fmt.Errorf("%w: %s", ErrNoServerFound, message)