Make it possible to provide a custom issuer for the certificate in the Helm chart

[bianchi2]

Description

It's a common practice to use one common issuer for all apps requiring certificates in a single cluster. This PR adds an optional certManager.existingIssuerName to values. When undefined, nothing changes and Issuer is created and referenced in the Certificate. When defined, no Issuer is created and a custom Issuer is referenced in the Certificate.

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the docs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes

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[k8s-ci-robot]

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This pull-request has been approved by: bianchi2
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[k8s-ci-robot]

Hi @bianchi2. Thanks for your PR.

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