To help avoid timing issues with autolaunch.json, when we are provided with a VM Service URI, we should support probing it from the extension to ensure it's accessible before we try to start a debug session. This way we can just start up later rather than failing if we react to the file before the service is fully accessible. We should make this opt-in with a flag:

{
  "type": "dart",
  "request": "attach",
  "name": "Attach to Flutter",
  "vmServiceUri": "ws://xxx",
  "waitForVmServiceMs": 30000, // number of ms to wait for vmServiceUri to become accessible

}

cc @keertip