Help flutter register callbacks on chrome proxy service

Flutter engine in native world registers `_flutter.listViews` extension on the VM. App registers extensions on the VM as well, and clients such as dart-code that are connected to flutter daemon call those extensions. Flutter daemon calls `_flutter.listViews` before proceeding to call methods registered by the app.

The web engine does not have information about the VM and its isolates so it cannot implement `_flutter.listViews` method that requires this information. For a short term solution, we are implementing `_flutter.listViews` directly in dwds, but it would be cleaner and less bug prone to implement a solution somewhere in flutter (in case that API changes, for example).

Possible solutions:
- sdk `developer` library can provide information about VM uri so flutter web engine can connect to it and
retrieve isolate information and implement `_flutter.listView`
- dwds can provide a hook to register service extensions directly on the chrome proxy service.

The issue names dwds, the Chrome proxy service, the SDK developer library, and the _flutter.listViews extension, but no files or tests. Start by tracing the existing _flutter.listViews handling in dwds and comparing the two proposed callback-registration approaches. Done means an agreed Flutter-side integration is documented and supports registering the required service callbacks.

Written by the indexing model from the issue text.