SyncConfig::subscription_initializer should pass in a threadsafe reference

[nirinchev]

Certain SDKs - such as dart - cannot invoke callbacks from random threads, which makes it impossible to use the subscription_initializer callback. I tried to add some locking code to have it execute on the dart thread, but then it's failing due to accessing the realm on an incorrect thread. Instead of passing in the Realm instance in the callback, it should pass in a thread safe reference, so the SDK can resolve it and obtain the instance on the correct thread.

[sync-by-unito]

➤ PM Bot commented:

Jira ticket: RCORE-2035