Add SDK/Example of OpenTAP Serializer plugin

It could be useful to extend the SDK/Examples with an example of a OpenTAP Serializer plugin.

Maybe it could be a serializer mapping this:

namespace OpenTap.Plugins.SomePlugin
{
    public class DelayMsStep : TestStep
    {
        public int DelayMs { get; set; }

        public DelayMsStep()
        {
            DelayMs = 1000;
        }

        public override void Run() { }
    }
}

To this:

namespace OpenTap.Plugins.OtherPlugin
{
    public class DelaySecStep : TestStep
    {
        public int DelaySec { get; set; }

        public DelaySecStep()
        {
            DelaySec = 1;
        }

        public override void Run() { }
    }
}