Add priorityClassName as an optional configuration

[mohsenasm]

With this PR, users can specify the priorityClassName for the pods:

apiVersion: crdb.cockroachlabs.com/v1alpha1
kind: CrdbCluster
metadata:
  name: cockroachdb
spec:
  priorityClassName: "a-priority-class-name"

Addressing Issue #1103.

Checklist

• I have added these changes to the changelog (or it's not applicable).

[cockroachlabs-cla-agent]

All committers have signed the CLA.

[mohsenasm]

@prafull01
Could you please review this? 🙏🏻

[prafull01]

Hi @mohsenasm Thank you for your contribution. Can you please rebase your branch with latest changes.

[mohsenasm]

Hi @prafull01, Thanks.
I updated the branch.

[prafull01]

Hi @mohsenasm Can you please look into failing tests. It is related to new field in crdbcluster CR.

Changes LGTM.

[mohsenasm]

Hi @prafull01, Could you please re-run the tests?

[mohsenasm]

@prafull01
Sorry, I missed the other CR file in the previous commit.