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/configuration/sources/secrets/openvpn.go

@@ -32,6 +32,22 @@ func readOpenVPN() (
 		return settings, fmt.Errorf("cannot read client key file: %w", err)
 	}
 
+	settings.EncryptedKey, err = readSecretFileAsStringPtr(
+		"OPENVPN_ENCRYPTED_KEY_SECRETFILE",
+		"/run/secrets/openvpn_encrypted_key",
+	)
+	if err != nil {
+		return settings, fmt.Errorf("reading encrypted key file: %w", err)
+	}
+
+	settings.KeyPassphrase, err = readSecretFileAsStringPtr(
+		"OPENVPN_KEY_PASSPHRASE_SECRETFILE",
+		"/run/secrets/openvpn_key_passphrase",
+	)
+	if err != nil {
+		return settings, fmt.Errorf("reading key passphrase file: %w", err)
+	}
+
 	settings.Cert, err = readSecretFileAsStringPtr(
 		"OPENVPN_CLIENTCRT_SECRETFILE",
 		"/run/secrets/openvpn_clientcrt",