Add GKE Shared VPC support

[danawillow]

All this needed was to allow setting the network/subnetwork to a self link.

WIP for now because I want to run the tests in CI. Tests are passing.

[morgante]

Does the test actually work? I believe that Kubernetes Engine also needs to be granted permission on the host network to work properly.

[danawillow]

Right, that's why it's still a WIP (Work In Progress)- I'm running the tests to figure out the minimum I can do for them, but our CI infrastructure requires running against a PR (there might be another way to do it but this is the easiest). Once I'm confident this is ready, I'll remove the WIP.

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

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