Adding existing secret

[cliedeman]

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• DCO signed
• Chart Version bumped
• [ x] Variables are documented in the README.md

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[carrodher]

Can you resolve the existing conflicts before continue?

[yeahphil]

Aside from the conflicts, this PR is incomplete. It doesn't change the secretKeyRefs in the chart, so rather than allow specifying a secret name, existingSecret is effectively just a flag that prevents creating a secret for the password.

That will work, but only if there's a secret with the name the chart would have used if existingSecret wasn't set (i.e. the output of template "postgresql.fullname" .).

I put together a PR that respects the value of existingSecret here: #9729

[cliedeman]

@yeahphil Thanks, I did that when I added this feature originally to the chart forgot to do it here.

[yeahphil]

@cliedeman thank you for the original work!