Describe your problem or use case

When we open a synced realm, there is no option to add subscriptions initially, right after the file is created and before the realm to be returned.

Describe the solution you'd like

There should be a place where the users can add subscriptions for synced Realm, before the realm to be fully downloaded. The SDK that provides Realm.openAsync implementation should receive this callback right after the realm file is created and before the download process has completed. We have to be sure that when the realm open async operation completes the data for the subscriptions to be downloaded.

Additional context

This callback have to be called only if the realm file is created for the first time.
There should be also a configuration option rerunOnOpen. If it is set to true the InitialSubscriptionsCallback should be called each time an application that is using realm SDK is launched.
Consider running two applications using the same Realm file and running one application twice.