Investigate how to enable callbacks invocation during garbage collection

[tustanivsky]

The way we currently handle beforeSend callback (and a few others) requires us to instantiate UObject-derived classes which could be unsafe during GC when outside of game thread (related issue for reference - #573).
Potential solution here could be switching to using USTRUCT's for entities that we pass to these callbacks but at the very least this is a major API change (example draft PR for breadcrumbs - #854).
Alternatively, we can explore the idea to expose some internal interfaces to plugin users and use them as callback params instead to avoid UObject instantiation entirely.