Add `--machine` support for `flutter attach` (#19077)

* Extract some of startApp into a reusable method

* Get basic attach --machine working

* Attach --machine tweaks

Move validation to validate method and create daemon early so we get the startup event before trying to get a connection.

* Bump daemon version so we know whether it's valid to flutter attach

* Tweak output text

* Swap package imports for relative

* Review tweaks (naming, formatting, typedefs)

* Separate arguments from process spawning

This will make calling attach easier

* Add a basic test for flutter attach --machine

* Fix crash if port unforward modifies the list of forwarded ports

* Add a no-op port forwarder for flutter-tester

* Switch to using BasicProject instead of our own inline code

* Fix expectation in test now we have a portForwarder

* Remove stale TODO (this is done)

* Tweak formatting

* Change some Completers to void to fix Dart 2 issues