Add support to use a GCP bucket instead of Minio as cache for the Gitlab Runner

[andypanix]

Minio is convenient, but in case you want to install more than one runner per cluster, an instance of Minio is installed for each runner.

An improvement would be to add an option to use an existing bucket instead of Minio.

The workload identity with which the runner runs should have permissions assigned to read and write to that bucket.