Add formal command-line args support to IActivity pattern about songhaycore

[BryanWilhite]

the current bright idea is to use the IActivityExtensions class to Parse() and DisplayHelp().

[BryanWilhite]

the latest bright idea is to pass args to the constructor of ActivitiesGetter to get Activity arguments and Activity name; the conventional help system should be part of ProgramArgs

[BryanWilhite]

This test, ShouldGetActivityFromDefaultName(), passes, addressing this issue.

Piggy-backed on this issue was coverage of the JsonConfiguration class for .NET Standard 2.0. Specifically, TraceSourceExtensions.EnsureTraceSource was added to optionally throw an exception when traceSource is null.

MSTEST2 Stuff

Note that the IConfigurationBuilderExtensions.WithSettingsJsonFile extension method looks for a basePath in the current directory or will turn to Environment.GetCommandLineArgs(). These conventions are not test friendly as MSTEST populates args with its own stuff. The workaround is to set our conventional app-settings.json to copy to the output directory.

One last MSTEST2 thing: since .NET Core does not support declaring trace listeners in configuration, I have to resort to stuff like this:

using (var listener = new TextWriterTraceListener(Console.Out))
{
    traceSource.Listeners.Add(listener);

    var args = new[]
    {
        nameof(GetHelloWorldActivity),
        "--world-name",
        "Saturn"
    };

    var getter = new MyActivitiesGetter(args);
    var activity = getter.GetActivity();
    Assert.IsNotNull(activity);

    activity.Start(getter.Args);

    listener.Flush();
}

Command-Line Help from ProgramArgs

Even though this issue is resolved, I would like to break the rules and tack on command-line help support around the recently added ProgramArgs.HelpSet property.