Add support for setting a default payment description in the gateway configuration

For anyone trying to make it work for INR/Stripe India

1. Go to commerce_stripe/src/Plugin/Commerce/PaymentGateway
2. Edit Stripe.php
3. Add Description like below

$default_intent_attributes = [
'amount' => $this->minorUnitsConverter->toMinorUnits($amount),
'currency' => strtolower($amount->getCurrencyCode()),
'payment_method_types' => ['card'],
'metadata' => [
'order_id' => $order->id(),
'store_id' => $order->getStoreId(),
],
'description' => 'Your Transaction Description',

I don't really see how the module would know when or how to set this value, but it wouldn't be a bad idea to make the description field available in the payment gateway settings. I propose the following:

1. Add an optional "Default payment description" / default_payment_description setting to both payment gateway configuration forms. Give it the help text, "Sent to Stripe when creating a payment intent. Required for some transaction types such as international payments."
2. If the gateway has a value for this setting, include it in the $default_intent_attributes.

Note: this description field can already be set from a custom event subscriber via the StripeEvents::PAYMENT_INTENT_CREATE, so we don't need any sort of alter hook here ... the module already supports adding custom properties to the payment intent.