[MISC] Remove kubectl interface

[Batalex]

This PR removes the deprecated kubectl interface.

I would appreciate extra care in reviewing the change done to spark8t/kube_interface/lightkube.py where I added the following

                        "data": {
                            k: base64.b64encode(v.encode()).decode()
                            for k, v in extra_args.items()
                        }
                        if extra_args
                        else None,

to the secret logic creation.
This was necessary to fix the test_service_account_get_config test since it previously used the kubectl interface to create the secret even if the fixture was using the lightkube interface.

An unintended consequence of removing this interface is that we can now be more precise in the type of the interface used, which means I had to fix a lot of type annotations to make the linter happy, sorry about that...

[theoctober19th]

Thanks for the much anticipated change!

I've also checked the slight change in the lightkube backend behavior when creating the secret, and yes -- the changes are sane, we probably overlooked that when we implemented lightkube initially.

[theoctober19th]

Looks good. Thanks for the changes!