Allow specifying different issuers for different Services

Copy of https://github.com/knative-extensions/net-certmanager/issues/353

> I'd like to have a way to use a different cert-manager issuer for particular services. This could be achieved in one of two ways:
>
> 1. overridden by an annotation on my service
> 2. multiple configs provided in config-certmanager with a label selector
>
> Option 1) seems easier to implement, although option 2) is more similar to how domain mapping is performed for overriding cluster-domain.

Generally we've been going with option 2 for configuring things (eg. domain and gateway selection)

Read the linked net-certmanager issue and the existing config-certmanager behavior first. Compare service annotations with multiple label-selected configurations, using the mentioned domain and gateway selection patterns as context. Done means services can select different issuers through a documented, tested configuration approach.

Written by the indexing model from the issue text.