Add client-side authentication features

[haijian-vaadin]

No description provided.

[Haprog]

So now that Middleware works as a bare function or class I get it that we need to call it as middleware.call(middleware, context, next), but imo it makes the code a bit confusing as in the class case the call method gets an unnecessary parameter that's not needed.

I think it would be cleaner to have a Middleware type which would be a union of two different types, where one is like the old function type and another is the new class interface and we can detect it to call them differently. This way anyone who creates a middleware class doesn't need to wonder why there's an extra parameter needed for the call method.

Otherwise this looks good except some typos I found.