Add support of priorityClassName to the helm chart

[mjshariati98]

The current helm chart doesn't support setting priorityClassName for the operator pods. By this PR, users can set priorityClassName as a helm value to be used by the operator.

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[bmsiegel]

Hi, it looks like this PR needs a rebase.

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PR needs rebase.

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[wallrj]

@mjshariati98 I've been writing some documentation explaining why to set priorityClassName when deploying cert-manager and what the value should be. I am interested to know what priority class name you intend to use for the aws-privateca-issuer in production and why.
Please read and feedback if you have time:

• Explain why to use priorityClassName: system-cluster-critical in production website#1444