Page instantiation: Productize prototype code for page and model activation

[Eilon]

Moved work item from #5357.

• IPageFactory and IPageActivator analogous to IRazorPageFactory and IRazorPageActivator.
• Model activation

The codes in question are here: https://github.com/aspnet/RazorPages/tree/master/src/Microsoft.AspNetCore.Mvc.RazorPages/Infrastructure

And they need to move to the MVC repo and gain some unit tests.