Fix deferred intent confirmation in the PaymentSheet Playground + Link (#4463)

## Summary
When using an alternate Link enabled mode, send the correct mode to the
example backend. If we don't do this, it will use the wrong merchant key
to attempt to access the PaymentMethod created by the client, which will
fail.

## Testing
In PS Example

## Changelog
None, PS Example

Author: davidme-stripe

Example/PaymentSheet Example/PaymentSheet Example/PlaygroundController.swift

@@ -826,6 +826,7 @@ extension PlaygroundController {
             "merchant_country_code": settings.merchantCountryCode.rawValue,
             "should_save_payment_method": shouldSavePaymentMethod,
             "mode": intentConfig.mode.requestBody,
+            "link_mode": settings.linkEnabledMode.rawValue,
             "return_url": configuration.returnURL ?? "",
         ] as [String: Any]