support remote debugging

[xinyanmsft]

I find the Java debugger quite helpful for local debugging. Thanks for the great work. I'd like to suggest 2 changes to enable remote debugging:

1. Allow user to specify host:port in launch.json file instead of hardcode to be localhost and a random free port.
2. Allow user to help you start the java program remotely as part of the pre-launch task.

[faustinoaq]

@xinyanmsft useful for this :point_right: #13

[scgm11]

+1