feat: add beforeSend hook

[ioannisj]

💡 Motivation and Context

Closes #155

Docs: PostHog/posthog.com#14482

Adds a beforeSend callback to PostHogConfig that lets you drop or modify events before they're sent to PostHog.

• Added to capture(), screen() and captureException()
• For $screen you can modify the screen name via the $screen_name property

Initially tried implementing this on the native side with method channels calling back to Dart in an attempt to also capture native initiated events, but hit timing issues since method channels are async but the beforeSend hook is called synchronously from native side. Also explored FFI but it added way too much complexity for the benefit.

For now, decided to keep it dart only so that it stays fast, synchronous and reliable. Should be good for the majority of the use cases.

Limitations in changelog

💚 How did you test it?

• Manual testing + modified sample project

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

[marandaneto]

left a few comments otherwise LGTM