Getting FLX subscriptions when FLX sync is disabled should be an error

[sync-by-unito]

At the object-store level, there should be a way to construct the SyncConfiguration in a way that indicates FLX sync is enabled and getting the subscriptions should fail if FLX sync is disabled.

[sync-by-unito]

➤ Michael Wilkerson-Barker commented:

• Updated Realm::get_latest_subscription_set() and Realm::get_active_subscription_set() to throw a runtime_error if called and flexible sync is not enabled.
• Updated "flx: no subscription store created for PBS app" test case to verify the runtime_error is thrown.

Pull request: #5637

[sync-by-unito]

➤ Michael Wilkerson-Barker commented:

• Updated Realm::get_latest_subscription_set() and Realm::get_active_subscription_set() to throw a runtime_error if called and flexible sync is not enabled.
• Updated "flx: no subscription store created for PBS app" test case to verify the runtime_error is thrown.
• Changes were reviewed and merged to 'master'