ENGCOM-5697: Resolve error when call getExchangeRates() and setExchangeRates(...) in Magento\Directory\Api\Data\ExchangeRateInterface #24272

 - Merge Pull Request #24272 from edenduong/magento2:2.3-bugfix/resolve_undefine_constant_exchange_rate
 - Merged commits:
   1. 3ee407a
   2. ac3cde1

Author: magento-engcom-team

app/code/Magento/Directory/Model/Data/ExchangeRate.php

@@ -5,6 +5,9 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+
+declare(strict_types=1);
+
 namespace Magento\Directory\Model\Data;
 
 /**
@@ -17,6 +20,7 @@ class ExchangeRate extends \Magento\Framework\Api\AbstractExtensibleObject imple
 {
     const KEY_CURRENCY_TO = 'currency_to';
     const KEY_RATE = 'rate';
+    private const KEY_EXCHANGE_RATES = 'exchange_rates';
 
     /**
      * @inheritDoc