When building the new SDK DAPs, this functionality was missed from the Flutter adapter (but added to the Dart CLI adapter).

As well as passing a VM Service URI directly, we support passing a file that can be polled for it.

This uses the same mechanism we use to obtain the VM Service URI from a Dart debug session (we run `dart --write-service-info=foo.json my_file.dart` and then poll that file which the VM will write) and is useful for users that have their own mechanism for launching an app (for example using custom Flutter embedders - see https://github.com/Dart-Code/Dart-Code/issues/3353) to provide a VM Service URI once the app is up and running.

Fixes https://github.com/Dart-Code/Dart-Code/issues/4577.