[HttpKernel] Extract method to instantiate controller in ControllerResolver

Replaces #10814 to merge into `master` instead of `2.3`.

---

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

Currently it's required to duplicate the entirety of the `getController()` and `createController()` methods just to replace the call to `new` (e.g. with container resolution, instead).

Now it's possible to just override the `instantiateController()` method.