Rewrite kubeflow-volumes-operator as sidecar

[misohu]

In this PR:

• Rewrote the operator using BaseCharm sidecar pattern.
• Rewrote unit tests to cover the new source code.
• The kubeflow manifests for cluster roles are now part of this charm. Make sure to approve this PR for kubeflow-roles-operator as the manifests are no longer needed there (previously we were not able to deploy those with podspec charm).

Because there are no proper integration tests as mentioned in #29, to test this PR I have deployed kubeflow-bundle 1.8 with this charm and I made sure I am able to

• Create a new volume in the volumes UI.
• I am able to use the newly created volume from inside the UI.
• I am able to remove this volume from UI.

CI note: publish is failing because I renamed the resource in charm. Will publish new oci image when approved <3

[orfeas-k]

Thank you @misohu. I have left some comments. I see that you followed patterns from other charms in implementing the CharmReconciler which helps with reviewing too, since those patterns have been already tested. I have a question though, did you use kubeflow-roles from PR when testing the charm?

[orfeas-k]

Good job @misohu, approved!