[local_auth] Convert Windows to Pigeon (#7012)
Updates local_auth_windows to use Pigeon, and moves to a more platform-tailored and Dart-centric implementation (rather than keeping the previous cross-platform method channel interface that the current implementation was duplicated from):
- Eliminates deviceSupportsBiometrics from the platform interface, since it's always the same as isDeviceSupported, in favor of doing that mapping in Dart.
- Eliminates getEnrolledBiometrics from the platform interface, since it was the same implementation as isDeviceSupported just with a different return value, in favor of doing that logic in Dart.
- Moves throwing for the biometricOnly option to the Dart side, simplifying the native logic.
Related changes:
- Adds a significant amount of previously-missing Dart unit test coverage.
- Removes the biometricOnly UI from the example app, since it will always fail.
Part of https://github.com/flutter/flutter/issues/117912