add ${aspnet-user-isAuthenticated}

[304NotModified]

[LayoutRenderer("aspnet-user-isAuthenticated")]
 public class IsAuthenticatedLayoutRenderer : AspNetLayoutRendererBase
 {
    protected override void DoAppend(StringBuilder builder, LogEventInfo logEvent)
    {
         var httpContext = HttpContextAccessor.HttpContext;
         if (httpContext == null)
         {
               return;
          }

        if (httpContext.User?.Identity?.IsAuthenticated)
        {
            builder.Append(1);
        }
        else
        {
            builder.Append(0);
        }
    }
}

todo

• non-core support

[snakefoot]

Not missing null-check of HttpContext like in #136? Also what if HttpContextAccessor returns null?

[304NotModified]

sharp @snakefoot ! Updated it.

[304NotModified]

https://github.com/NLog/NLog/wiki/AspNet-User-isAuthenticated-Layout-Renderer