Added '--check-for-remote-artifacts' option for Flutter Doctor. (#23543)

* Added '--check-for-remote-artifacts' option for Flutter Doctor.

This option takes a Flutter engine revision and issues HEAD requests to
determine whether or not artifacts for the provided engine revision are
available from cloud storage. This functionality is needed for the Dart
SDK autoroller to avoid creating a PR to roll the engine into the
framework while artifacts haven't finished building, which would cause
Cirrus tests to fail.