Whether the app can present the external notice sheet.
Check this property before showing any UI for external purchases
Check the value of this property again and update your UI if
`Storefront.current` changes.
- Important: If this property is `false`, do not call `presentNoticeSheet()` as the method
will always fail.
Present a notice sheet to users before showing external purchases.
Only call this method as a result of deliberate user interaction, such as tapping a button.
- Returns: Whether the user chose to continue to view the external purchases. Only show
external purchases if the result is `NoticeResult.continuedWithExternalPurchaseToken`
or `NoticeResult.continued`.
- Throws: A `StoreKitError`