Add DNSConfig support for GKE Clusters

[jensentanlo]

What problem are you facing?

Currently this field is not exposed in the cluster spec.
When I pulled the repo, I noticed that the gcp go client version is quite old and is missing this field, so we'd need to upgrade the client but it's there in the newer versions of the client.

Relevant REST doc for the field:
https://cloud.google.com/kubernetes-engine/docs/reference/rest/v1/projects.locations.clusters#Cluster.DNSConfig

How could Crossplane help solve your problem?

It would be great if this field could be added.

[jensentanlo]

Just a note but I'll be tackling this (if this feature is okay to add)

[Feggah]

Thanks for raising this issue, @jensentanlo!

Go for it 🙂 feel free to ask for my review when you open the PR