Reuse existing (Cluster)Issuer for webhooks certificate

[Pionerd]

Issue

N/A

Description of changes

Instead of always creating a self-signed Issuer when opting to use cert-manager for the web hooks certificate, I would like to add the possibility to reuse an existing (Cluster)Issuer.

Checklist

• Added/modified documentation as required (such as the README.md for modified charts)
• Incremented the chart version in Chart.yaml for the modified chart(s)
• Manually tested. Describe what testing was done in the testing section below
• Make sure the title of the PR is a good description that can go into the release notes

Testing

Verified helm templating with different options
Deployed locally and tested the different options (with/without certmanager, with/without own Issuer)

Remark: the existing enableCertmanager value is changed to certManager.enabled (since other options are added as well). Don't know if you guys expect a patch or minor upgrade of the helm chart for this one.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.