SyncSession Scaffolding

[sync-by-unito]

Add the initial wrapper implementations for SyncManager and SyncSession. They do not need to expose anything interesting:

// SyncManager class
class io.realm.mongodb.sync.Sync {
  // ...
}

// Session class
class io.realm.mongodb.sync.SyncSession {
 // ...
}

val Realm.syncSession // Extension method on Synced Realms

[cmelchior]

For now, there doesn't seem to be a use case for a io.realm.mongodb.sync.Sync class. All functionality has been folded into the SyncSession class.