Allow `beforeSend` in the Flutter layer to apply to native events

[buenaflor]

Description

Currently events in Dart and Native are handled separately and only few things are synced such as contexts, breadcrumb etc...

To further enhance the experience we can implement the beforeSend in such a way so it is also applied to native events which for example could allow custom fingerprints to be added or certain native events to be dropped from the Dart layer.

Now that we use FFI/JNI to communicate with native APIs this might be not be as difficult to implement as before with method channels

This should be gated behind a flag (name TBD) and disabled by default (opt-in)

Example of real use cases

• There's no way to customize the fingerprint when an ANR occurs. #3100

[linear]

DART-221 Allow `beforeSend` to apply to native events