`attachThreads` should attach native threads even if the exception comes from Dart/Flutter

[ueman]

If I enable the attachThreads option, I would expect that native threads are attached to my crash-reports, even if the crash reports come out of the Dart/Flutter layer.

One tricky issue, I can imagine, is that the timing of the reading of the active threads plays a significant role. Ideally, the native threads are read at the time when Sentry.capture* is called. Otherwise, it might not become as useful.

Other problems include: How do I see what's a native thread, and what's a Dart/Flutter thread (isolate).

### Blocked by
- [ ] https://github.com/getsentry/sentry-cocoa/issues/1870

[philipphofmann]

Negative aspects of this issue:

1. Getting the stacktraces of all threads from the native layers adds some overhead.
2. Most of the time native threads won't help you to understand bugs in the Dart layer.

This is already possible on Android, cause it has attachThreads. On Cocoa, we have the basic functionality since App Hangs, but no optionattachThreads yet https://github.com/getsentry/sentry-cocoa/issues/1870.

Of course, this feature could be useful for some issues.