Performance optimization. Ability to specify payer when doing transaction
reservation instead of doing one more API call to get payer's walletId,
before performing reservation.

Example of usage:
$payer = Paysera_WalletApi_Entity_WalletIdentifier::create()->setCard(
    Paysera_WalletApi_Entity_Card::create()
        ->setIssuer('vilniecio')
        ->setNumber(123456789)
);

$api->acceptTransactionUsingPin('transaction_key', $payer, '1234');