Implement before-send hook

[marandaneto]

Summary

Implement the canonical before-send hook behavior from sdk-specs for the Unity SDK.

Expected behavior

• Apps can register a hook that receives the assembled event before enqueue/send.
• The hook can mutate event properties.
• The hook can drop an event.
• Hook exceptions should not crash capture callers; the SDK should record/log a warning.

Spec

https://github.com/PostHog/sdk-specs/blob/main/acceptance/private/before-send-hook.feature

Notes

This should be implemented in a Unity-friendly way and work consistently across supported Unity platforms.