Add support for existing external secrets

[ccremer]

Summary

• New parameter webhook.externalSecretName (skips rendering of included Secret if given)
• Note: Setting the certificates in the webhook.{privateKey,caBundle,certificate} is still required, though if there are means to patch the caBundle field through other 3rd party tools, that is acceptable as well. In thoses cases, webhook.caBundle can be left empty and it will not be rendered in the template.
• Closes #70

Checklist

For Helm Chart changes

• [x] Categorize the PR by setting a good title and adding one of the labels: bug, enhancement, documentation, change, breaking, dependency as they show up in the changelog
• [x] PR contains the label area:chart
• [x] PR contains the chart label, e.g. chart:provider-postgresql
• [x] Variables are documented in the values.yaml using the format required by Helm-Docs.
• [x] Chart Version bumped if immediate release after merging is planned
• [x] I have run make chart-docs
• [x] Link this PR to related code release or other issues.

[ccremer]

I renamed the value to webhook.externalSecretName, to be more clear