Helm chart: Support using a pre-existing Issuer to generate the webhook's certificate

[nogweii]

This is a small feature request, that I can work on at some point in the future but don't have time right now. It's pretty simple - make the creation of a new self-signed root optional, and allow someone to provide the name of a custom Issuer (or even ClusterIssuer?).

I'd like this as I'm trying to unify all of the cluster webhooks to use the same root CA, rather than have a proliferation of one-offs for each individual one.

Thoughts?

[zachomedia]

@nogweii Thanks for submitting this, I would definitely accept this as a contribution if submitted.