Consider how to consistently configure Serving and Eventing encryption issuers

[pierDipi]

Problem

We need to think about how to consistently configure Eventing and Serving encryption issuers when users want to change the issuer used.

Serving has this configuration into a ConfigMap while Eventing ships with actual cert-manager issuers, so the usual spec.config wouldn't work for both and ideally we have a consistent API for both

Persona: Which persona is this feature for? Administrator

Exit Criteria A measurable (binary) test that would indicate that the problem has been resolved.

Align the configuration for serving (net-certmanager) and Eventing issuers

Additional context (optional)

/area API