Offer option to create private functions

[rhuss]

You can deploy Knative Services so that they are not exposed to the outside of the cluster: https://knative.dev/docs/serving/services/private-services/

This is especially useful when building up an event mesh where the function is just reacting on events.

It would be great if func could offer an option to create the necessary labels on the deployed Knative Service to shield the user from the internal mechanics. If in the future func should also work with vanilla K8s Deployments and Ingress, this option would just create a Deployment and K8s Service, no Ingress, so its not necessarily Knative service specific.