For methods like ::crd and ::api_resource (see #495). Came up on discord last week, and it'll be useful to keep some guarantee on the generated code.

• define trait somewhere in kube::api - object or new file under src/api
• impl CustomResourceExt in kube-derive

cc @lfrancke.

With #317 the tests for the controller were removed. We need to add them back against the updated framework.