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.

Start by reading AutoRouterImproved and the parent router methods it currently uses for finding the controller and method name. Compare the two proposed approaches: reusing parent-router behavior or making auto routers replaceable and extendable through app-specific services. Done requires a decided design and an implementation that supports changing the controller and method name without duplicating the router.

Written by the indexing model from the issue text.