refactor: merge community operator packages

[lsierant]

Summary

This is a no-op refactor.

• Merged mongodb-community-operator/pkg/kube/statefulset into pkg/statefulset. All other mongodb-community-operator/pkg/kube packages should be moved the same way, but this PR doesn't touch them.
  • some merged unit tests had the same names; conflicting names from MCO were renamed with _MCO suffix.
• extracted general resource and status functions from controllers/operator into pkg/kube/commoncontroller so that the functions can be shared by other controllers that are in different packages

Proof of Work

Green tests.

Checklist

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