Add support for by-ref-like (ref struct) parameter types such as Span<T> and ReadOnlySpan<T>

[stakx]

Unlike my earlier draft #664, this PR will not require DynamicProxy user code to set up any value converters for byref-like parameters. Instead, any byref-like argument will automatically get wrapped in one of the new types ByRefLikeArgument, ByRefLikeArgument<TByRefLike> (only available on .NET 9+), SpanArgument<T>, or ReadOnlySpanArgument<T>. The original byref argument value can be retrieved from those wrappers using their Get or GetPointer methods. The latter retrieval method hints at this new feature involving some unmanaged pointer trickery under the hood... but it can probably be made reasonably safe to use. (I'll make sure to write a documentation page for this new feature so that there's some guidance.)

public interface IFoo
{
    void Method(in Span<char> characters);
}

public void FooMethodInterceptor : IInterceptor
{
    public void Intercept(IInvocation invocation);
    {
        Span<char> characters = (invocation.Arguments[0] as SpanArgument<char>).Get();
    }
}

I'm not done yet, so marking this PR as a draft. I'll add a more complete description for the new API as I make further progress.

Will close #663 once completed and merged.