Add CreateTagHelper to RazorPage.

- The CreateTagHelper method is responsible for creating and activating TagHelpers.
- Added the support for requesting the ViewContext.
- Added tests to validate the tag helper creation mechanism.

#1104
You could simply cast the instance here.

```C#
var canHasViewContext = tagHelper as ICanHasViewContext;
canHasViewContext?.Contextualize(ViewContext);
```