code: public function listSubscriptionsByCustomer(string $customerId)
      {
          $cacheKey = sprintf('zoho_subscriptions_%s', $customerId);
          $hit = $this->getFromCache($cacheKey);
  
          if (false === $hit) {
              $response = $this->sendRequest('GET', sprintf('subscriptions?customer_id=%s', $customerId));
  
              $result = $this->processResponse($response);
  
              $invoices = $result['subscriptions'];
  
              $this->saveToCache($cacheKey, $invoices);
  
              return $invoices;
          }
  
          return $hit;
      }
docs: @param string $customerId The customer's id
  
  @throws \Exception
  
  @return array