Allow selection of existing GKE clusters in Cluster page

Description

We already support connecting to GKE to create a new cluster. Why not allowing to select one of the existing clusters instead?

Proposal

In the CI/CD > Cluster page, allow selection of existing GKE clusters fetched realtime from GCP.

The flow could be:

1. ask for OAuth2 if not already available, to interact with GCP
2. fetch the list of GKE clusters user has access to, via GCP APIs
3. list the clusters in a dropdown
4. user picks up one
5. fetch parameters for that cluster, configure it as an existing cluster

Links / references

Kubernetes/GKE integration: https://gitlab.com/gitlab-org/gitlab-ce/issues/35956