apim-apps Fix #3657: Set default Subscription Plan during Async API creation

Issue Reference

Description

When creating an Async API by providing a URL or uploading a file, the subscription plan was set to null. This caused issues where no default subscription policy was applied in the deployment.

Fix

Set the default subscription policy to ['AsyncUnlimited'], ensuring that every Async API has a default subscription policy unless explicitly changed by the user.
This implementation follows the same approach as the REST API, where a default subscription policy is assigned unless a different value is explicitly set.