Lightweight Big Bang Core example that expects Istio's cert to already be present

[RothAndrew]

Quick persona:
I'm Joe Schmoe. I'm a user of Zarf who is using it to deploy Big Bang, but I don't want my TLS certificate to be in the main package, since I want to make the zarf package freely available, but need to protect the cert as a secret.

Story:
As Joe Schmoe, I want an example Zarf package that I can use as a reference to create a Big Bang deployment that expects the ingressgateway's certificate to already exist in the correct kubernetes secret in the correct namespace, so that I can provide my private secret separately from the zarf package.

[RothAndrew]

Customer requested to escalate the priority of this, since it's his exact use case he's working on right now.

/cc @YrrepNoj @matt-strong @jeff-mccoy

[RothAndrew]

Moving this back to Ready to Start for now since it is being preempted by other priorities