Improve the GKE pipe to support private clusters (accessed through a bastion)

The pipe for GKE (https://bitbucket.org/product/features/pipelines/integrations?p=atlassian/google-gke-kubectl-run) works well on public clusters, but not on private ones.

For private GKE clusters, you need to first open an SSH tunnel through a bastion, and then use it as an HTTPS proxy for kubectl (see https://cloud.google.com/kubernetes-engine/docs/tutorials/private-cluster-bastion)

It's certainly possible to do that manually with a script.

But it would be way better if your built-in pipe supported it.

NB: original question was https://community.atlassian.com/t5/Bitbucket-questions/Could-the-GKE-pipe-be-improved-to-support-private-clusters/qaq-p/2158869