Run on same thread in generic host

[vlm---]

I was playing with new version and run into some trouble because my code wasn't executed on UI thread. It lead to code in https://github.com/natemcmaster/CommandLineUtils/blob/master/src/Hosting.CommandLine/Internal/CommandLineService.cs class and this method:

public Task<int> RunAsync(CancellationToken cancellationToken)
{
     _logger.LogDebug("Running");
     return Task.Run(() => _state.ExitCode = _application.Execute(_state.Arguments), cancellationToken);
}

It uses Task.Run to execute application login on different thread which is not desired in certain scenarios where you want your code to execute on the UI thread.
It can be easily rewritten to:

public Task<int> RunAsync(CancellationToken cancellationToken)
{
     _logger.LogDebug("Running");
     _state.ExitCode = _application.Execute(_state.Arguments);
     return Task.FromResult(_state.ExitCode);
}

That way the application logic is always executed on same thread. Would you be interested in PR for this change?

[natemcmaster]

Sure, I'd take a PR. @lucastheisen would this change be fine?

[lucastheisen]

No objection here.

Honestly, I don't have any UI use cases, so have no idea what the implications would be there. I don't imagine it would hurt anything.

[TheConstructor]

@vlm would this also work? #196

[vlm---]

It should work and would be better because async should be all the way from top to bottom and Task.FromResult is just like any other task (some results are internally cached), see first chapter in https://blogs.msdn.microsoft.com/dotnet/2018/11/07/understanding-the-whys-whats-and-whens-of-valuetask/ for better explanation. No need for my PR anymore.