chore: move k8s specific code to its own class

[gruyaume]

Description

The AMF has k8s specific code used to create/remove the NGAP service. It used to be entangled in the charm class. Here , we move this k8s specific code to its own class and file. This separates concerns and move the dependencies closer to their actual use. This also allows for simplifying unit tests.

Here we did not write unit tests for this new class as it is a thin layer with close to 0 logic.

Checklist:

• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that validate the behaviour of the software
• [ ] I validated that new and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules
• [ ] I have bumped the version of the library