Support error tracking and APM for `dart_frog`

### Description

https://pub.dev/packages/dart_frog

Dart Frog is a server-side framework, similar to NodeJS and friends.

* Consider adding a header util instead of parsing headers manually, [issue](https://github.com/getsentry/sentry-dart/issues/1384).
* Check if the single hub/scope won't attach spans to the wrong transaction, this could be possible due to the nature of the Dart SDK being user SDKs as of now (Mobile, Web).

If you're interested in this, please upvote this issue!