GKE Cluster Source

[ribeiro-rodrigo]

The purpose of this task is to implement support for the GKE (Google Kubernetes Engine) cluster source.

The access information as a service account token will be passed on to the secret that the cluster source must know how to read.

The custom resource that contains the GKE source cluster should look like this:

  source:
    secretName: gke-secret
    gke:
      filters: {}
      tags: {}
      ignore:
        - cluster1
        - cluster2
        - cluster3 

The cluster source must allow in the filters field, all filters allowed in the command gcloud container clusters list.
The rules for reading the secret performed by the cluster source must follow the requirements specified in issue #4.