[Feature request] Expose UNTimeIntervalNotificationTrigger with no repeat

[Albert221]

Hey! Currently, the plugin doesn't allow taking advantage of UNTimeIntervalNotificationTrigger with the repeat set to false and it would be really nice to have this possibility exposed within the Flutter plugin.

The plugin currently exposes periodicallyShow that uses UNTimeIntervalNotificationTrigger with the repeat true.

My use-case is using the plugin for showing a notification on closing the app. It works natively by running a scheduled notification with time interval trigger and no repeat, but does not by other means (it can also be safely called from Flutter side during app exiting so no problem on that part), but the plugin doesn't expose that.

[MaikuB]

Could you look into a submitting a PR for this?