AutoRouterImproved extendability

In the current state, the `AutoRouterImproved` is not thought to be replaced by an App-Specific service and to be extended on.

>[kenjis](https://github.com/codeigniter4/CodeIgniter4/issues/5889#issuecomment-1476185230): Because it is better to implement the interface, not extending the class.

In my case, I need to change the controller & method name which is already implemented in the router.
Re-Implementing the `AutoRouterImproved` to do that seems kinda ridiculous.

Two approaches come to my mind:

1. The specific solution: Use the "parent" routers methods for finding the method and controller name, this prevents duplicate code and allows for a little app-specific behavior.
2. The generic solution: Make the auto routers replaceable by app-specific services
1. Optional: make both extendable, so we don't have to write ridiculous amounts of code for small changes.