GCP - Support private API and operator load balancers

[deliahu]

Here is the guide for private load balancers

[HodorTheCoder]

Let me know how I can help with this. I am trying to find the options in gke.go and online to add to make this work. Thanks.

[deliahu]

(just posting here in case anyone else is following the thread, since @HodorTheCoder and I connected via email)

I've created this branch with a temporary implementation to test the feature.