Negotiate reflection performance improvements

Feedback from @mconnew:

I was looking over your reflected abstraction of the negotiate code ... and noticed a significant performance problem in the code. There is lots of code which looks similar to this:

public bool IsCompleted
{
get => (bool)_isCompleted.Invoke(_instance, Array.Empty());
}

The problem with calling the Invoke method on an MethodInfo is it does a whole bunch of runtime type checking to make sure the number and type of arguments is correct. There’s also a boxing and unboxing as well as a cast type check on the return value. You can use MethodInfo.CreateDelegate to create a delegate you can straight up call. Because it only does the type checking of the delegate type to the method info when you call CreateDelegate, it’s a lot cheaper to call. The generated delegate is strongly typed so the CLR just depends on the strong typing to ensure all the parameters etc are correct.

---

If we get a proper API for this in 6.0 then this code can be removed (https://github.com/dotnet/runtime/issues/29270). If not we may consider making some improvements.

Note it's not clear how these costs reflect on RPS. Windows Auth is already a very in-efficient process that often involves multiple request.