feat(payments-next): Add free trial support to checkout flow

Because:

* Users eligible for free trials should receive trial subscriptions when checking out via Stripe or PayPal
* Subscription management flows need to handle 'trialing' status for cancel and resubscribe actions

This commit:

* Integrates getFreeTrialEligibility with existing payWithStripe and payWithPaypal methods
* Records a free trial record with firestore
* fixes bug with strapi country-code checks
* updates subscription management page to allow for cancellations of 'trialing' subscriptions
* Adds tests for new and existing cases

Closes #PAY-3543

Author: david1alvarez

libs/payments/cart/src/lib/checkout.error.ts

@@ -253,3 +253,17 @@ export class IntentInsufficientFundsError extends IntentFailedHandledError {
     this.name = 'IntentInsufficientFundsError';
   }
 }
+
+export class UnexpectedSubscriptionStatusForTrialError extends CheckoutError {
+  constructor(cartId: string, subscriptionId: string, actualStatus: string) {
+    super(
+      'Expected subscription status to be trialing for free trial checkout',
+      {
+        cartId,
+        subscriptionId,
+        actualStatus,
+      }
+    );
+    this.name = 'UnexpectedSubscriptionStatusForTrialError';
+  }
+}