Use LoadBalancer Service

[DanielArndt]

Description

Exposes an external LoadBalancer service for the UPF, similar to canonical/sdcore-amf-k8s-operator#24 for the AMF

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

[Gmerold]

Two general comments:

1. I think we could mention in the README that it is possible to expose UPF, but LB is required for that. We could also have an integration test validating the exposure. WDYT?
2. Please add docstrings :)

[DanielArndt]

Two general comments:

1. I think we could mention in the README that it is possible to expose UPF, but LB is required for that. We could also have an integration test validating the exposure. WDYT?

Sure, I can add that to the README.

I'd be happy to give integration tests a shot, but at this point I'm not quite sure what that would look like. I'll have to tackle those when I get back on Wednesday.

2. Please add docstrings :)

Happy to add docstrings where you think they would be useful. I'm not a fan of docstrings on internal functions where the docstring just reiterates the function name. i.e.

def _create_external_upf_service(self) -> None:
    """Creates the external UPF service."""

The above seems like a waste, but maybe there is something you'd like me to be more explicit about?

[gatici]

Needs to be rebased before merging. LGTM.