Implement the "dotNetCliPaths" option to support custom .NET SDK locations

[jkoritzinsky]

Depends on https://github.com/OmniSharp/omnisharp-roslyn/pull/2227

[SamB]

... don't you need to register the option in package.json ?

[jkoritzinsky]

@SamB I did not realize I needed to do that. Done!

[JoeRobich]

@jkoritzinsky The c# extension also performs a check for dotnet to be on the PATH. Now that we are adding the ability to specify additional paths, can you update the getDotnetInfo function to return results when dotnet isn't on the path but instead specified by the new setting.

[gregg-miskelly]

The debugger also needs to find dotnet. So this directory probably needs to be added to PATH through DebugAdapterExecutable.options.env in createDebugAdapterDescriptor.

There are also build tasks that need to use dotnet, but I am not sure if there is a good way to add that to the PATH that VS Code uses, or if users would need to manually edit tasks.json.

[jkoritzinsky]

@gregg-miskelly @JoeRobich I've finally had a chance to get back to this PR. I've updated it to master and implemented both of your requests. Can you take another look when you have a chance?

[jkoritzinsky]

@JoeRobich @filipw can one of you merge this PR for me? Or is there something else I need to do before we get this merged in?

[JoeRobich]

@jkoritzinsky Sorry for the delay. Thanks again!