Add a `login` subcommand

[flavio]

Right now kwctl reuses the docker credentials to be able to pull and push to OCI registries. However, what if the user doesn't have the credentials for a certain registry?

Acceptance criteria

• kwctl keeps looking for the docker credentials, and uses them when available
• kwctl login creates its own credential files, with the username/password of the registry that was provided to the login subcommand