Sidebar doesn't work in web contexts because DTD URI is not exposed

[DanTup]

This is a part of the problem for #5158. DevTools now needs a connection to DTD, but when running in a web context, it tries to connect to localhost (but the service is running on a remote container).

With flutter/devtools@5457f57, we can pass a second DTD URI to DevTools, but we need to expose and provide it here in the extension.

[DanTup]

Dart 3.6.0-255.0.dev includes the new CLI option and has rolled into Flutter master.