Support running command before attaching debugger

It's possible to debug tests running from a Maven build by calling mvn -Dmaven.surefire.debug test, then attaching the debugger to a remote Java process on port 5005, like

{
    "type": "java",
    "name": "Debug (Attach) to Maven tests",
    "request": "attach",
    "hostName": "localhost",
    "port": 5005
}

So this requires 2 separate actions from the user. Ideally, one would define a new 'surefire-debug' task for mvn -Dmaven.surefire.debug test and add "preLaunchTask": "surefire-debug" to the launch configuration. The problem is running that command doesn't return an exit code, since it's waiting for a debugger to attach, so the prelaunch task never finishes and the debugger never kicks in: you're stuck.
So it'd be nice if the debugger detected port 5005 is responding, before the preLaunchTask finishes.
If preLaunchTasks absolutely must finish before the debugger kicks in, then we might need to have a preLaunchCommand (or whatever better name) instead. Debugger bootstrap waits until port 5005 responds following execution of preLaunchCommand, then proceeds.