Switch the existing Sentry classes to use USTRUCTs

[tustanivsky]

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## Unreleased

- Switch the existing Sentry classes to use USTRUCTs ([#854](https://github.com/getsentry/sentry-unreal/pull/854))

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[nwhite-riot]

In general, it seems odd that we ever expose this. Perhaps we can remove the need for this and any public callers can use a different method? For example, we use GetNativeObject() to call sentry_value_set_stacktrace - perhaps instead we have a method on the the Sentry event called AddStacktrace and that calls the method underneath?

[tustanivsky]

That's a good point. There are definitely a few use cases where we can remove GetNativeObject() and route calls to underlying SDKs through our Unreal wrappers directly. I'll look into this and see if it's something we can achieve.