Add option to control automatic setting of Gateway on Orders

[rynpsc]

Currently when creating an Order (specifically before validating an Order) a default Payment Gateway is set.

commerce/src/elements/Order.php

Lines 1238 to 1243 in d4d51cb

	if (!$this->gatewayId && !$this->paymentSourceId) {
	$gateways = Plugin::getInstance()->getGateways()->getAllCustomerEnabledGateways();
	if (count($gateways)) {
	$this->gatewayId = key($gateways);
	}
	}

It would be nice to have the option to opt out of this behaviour in the same way you can control the automatic setting of the Shipping Method via the autoSetCartShippingMethodOption setting.

One use case for this would be in a checkout flow whereby a summary of the progress through the checkout is displayed across multiple pages based on what information has been set in the checkout flow. W wouldn't want to show a payment method until the user had reached that step and set one.

We currently work around this by checking what page the user is on but would be nice to just be able to do {% if cart.gatewayId %}.

Another use case would be where the user can checkout without paying, in that scenario if would be good to be able to "clear" the gateway from the order.