Add option to keep the Debug Server always running

[sankasan]

Summary:
It would be very helpful if the user could start a debug server and keep it running for the time VSCode keeps running. This allows external processes to initiate the launch of the script while still connecting to the debug server in VSCode.

Example case:
Currently I'm working on a C++ API for python. This C++ API ends up in a .dll which python scripts load. In Visual Studio 19` I set launch options to call the python interpreter with the script. This allows me to debug the API on the C++ side. If the debug server would keep running on the VSCode side, and let the script connect the the python debugger in VSCode when initiated from Visual Studio, I would be able to hit breakpoints in both C++ (Visual Studio) and Python (VSCode).

It might be that this feature already exists but I couldn't find it in the documentation.

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For what its worth, pydevd in Eclipse has this feature, and this keeps me attached to that platform. It is simply too powerful of a development feature.

Below some screenshots of the feature in pydev as exposed in Eclipse.

[int19h]

Something like this should be possible if you combine "attach" / "listen" with "restart": true in the debug configuration - this causes VSCode (or any other client that supports restarting) to spin up the debug adapter on IDE side again every time the session ends, so effectively you end up with a permanent listener that can be connected to via debugpy.connect() as needed.