Add AsyncTaskCodeActivity

[Foxtrek64]

Fixes #145

• Define VoidResult type (represents a void type since you can't return System.Void)
• Define AsyncTaskCodeActivity type
• Write unit tests
  • VoidResultTests
  • AsyncTaskCodeActivityTests

Usage Example:

public class DownloadWebPage : AsyncTaskCodeActivity<string>
{
    [RequiredArgument]
    [OverloadGroup("Address")]
    public InArgument<string> Url { get; set; }

    [RequiredArgument]
    [OverloadGroup("Address")]
    public InArgument<Uri> Uri { get; set; }

    protected override async Task<string> ExecuteAsync
    (
        AsyncCodeActivityContext context,
        CancellationToken cancellationToken
    )
    {
        var url = Url.Get(context);
        var uri = Uri.Get(context);

        var path = url ?? uri.AbsoluteUri;

        return await Download(path, cancellationToken);
    }

    private async Task<string> Download(string path, CancellationToken cancellationToken)
    {
        IFlurlResponse response = await path.GetAsync(cancellationToken);

        switch (response.StatusCode)
        {
            case 200:
                return await response.ResponseMessage.Content.ReadAsStringAsync();
            case 404:
                return "404: Not Found";
            default:
                return $"{response.StatusCode}: {response.ResponseMessage.ReasonPhrase}";
        }
    }
}

[Foxtrek64]

PR is complete. Let me know if there are any changes you would like for me to make.

Also, sorry for the delay on this.

[github-actions]

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