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 (related issue for reference - #573). In order to prevent potential issues in this situation we're skipping callback invocation (if one is configured) entirely during GC.

[tustanivsky]

Utility allowing to reproduce this behavior:

• Add demo app utility allowing to capture event during garbage collection #1038

[tustanivsky]

What options do we have:

1. Don't use UObjects in callback handler interface

We can expose certain internal interfaces (e.g. ISentryEvent) to plugin users and use them as callback parameters instead of UObjects. This would mean operating on plain C++ objects which would not be accessible from Blueprints.

An alternative is to switch to using UStructs for the entities passed to the callback handler. However, this would be a significant API change (see the old draft PR for breadcrumbs: #854).

2. Pre-allocate UObjects pool

Required UObjects can be created during initialization and reused during garbage collection. Before invoking the callback, the underlying native objects can be assigned to them via setter function.

3. Leave current implementation

The existing limitation is documented and also mentioned in the repo README so we can leave it as-is for now - at least until it becomes an issue for a larger number of users or the engine allows bypassing it in newer versions.

4. Other considerations

• Waiting for GC to finish before invoking the callback can introduce potential timeout or deadlock risks
• Non-fatal events occurring during GC can be queued and re-captured afterward but this does not address the issue for crashes