Allow for a payment gateway object when creating a payment

Currently the payment storage does not allow creating a payment with a payment gateway object:

    $payment_gateway = $this->order->payment_gateway->entity;
    $payment_storage = $this->entityTypeManager->getStorage('commerce_payment');
    $payment = $payment_storage->create([
      'state' => 'new',
      'amount' => $this->order->getTotalPrice(),
      'payment_gateway' => $payment_gateway, // Not allowed
      'order_id' => $this->order->id(),
    ]);