Further work on CockroachDB Version Validator

To DO for PR https://github.com/cockroachdb/cockroach-operator/pull/320 :

1. See @chrisseto and @chrislovecnm suggestions on version checker:
- These errors could be avoided by instead checking the pod's readiness or using the log method described above. Detecting errors based on strings is pretty flakey.
- If we're execing into the job pod, what is the job pod doing? Would it make more sense for the job to run the above command and then have this code parse the output via the logs API?
- IsJobPodRunning function is misleading, it doesn't actually wait for the pod to report itself as ready. There's a helper in the k8s SDK to check the readiness of a pod. You could also use a field selector.
- I find it helpful to make Unknown the zero value for these types of enums. As is, it wouldn't be possible to tell the difference between an uninitialized value and Failed which might result in some strange bugs.
2. Add refactory for Actor method Handles
3. Add SupportedCrdbImages tests

Start by reviewing PR 320 and the version checker discussion, including the readiness and logs API suggestions. Inspect IsJobPodRunning, Actor Handles, and SupportedCrdbImages; done means resolving the validator design questions, refactoring Handles, and adding SupportedCrdbImages tests.

Written by the indexing model from the issue text.