VSCode should provide an API to allow extensions to launch a debug adapter

[gregg-miskelly]

In order to enable the C# extension to be able to launch the debugger against unit tests, we need to allow the many body of the C# extension to be able to start the debugger, and we need some way to tell the debugger what to do (in our case attach to a specific process).

To solve this, we would like there to be an API exposed to extensions which allows launching the debugger with a specified set of launch option json. Something like:

export function launchDebugger(launchOptions :any) : Thenable

[jrieken]

@isidorn Isn't there a command for this? Then executeCommand should do it

[isidorn]

Dup of #4615