Potential memory leak

[Roshyo]

In Setono\SyliusGiftCardPlugin\Controller\Action\GiftCardBalanceAction controller (and same in API when merged to master), we have a performance issue regarding huge amount of GiftCards due to this code block:

$giftCardBalanceCollection = GiftCardBalanceCollection::createFromGiftCards(
    $this->giftCardRepository->findEnabled()
);

We should come up with a solution to avoid that.