Allow extensions to handle launching of the debug adapter.

[vadimcn]

It would be nice if extensions could be in the loop when launching the debug adapter sub-process:

• This would allow the extension to alter adapter's command line options depending on workspace configuration and/or pass the relevant bits of configuration to the adapter.
• In cases when adapter is implemented on top of system debugger, this would allow the extension to locate a proper binary (for example, lldb could be installed as lldb.X.Y on some systems).
• Extension could provide better error messages should the adapter exit abnormally.

[weinand]

@isidorn this feature request makes a lot of sense and would bring debug-adapters closer to language servers.

[isidorn]

Setting to November to investigate