PGO chart: Contribute GCS configuration to upstream so we can use that as a base for the chart

[pbastia]

Upstream: https://github.com/bcgov/crunchy-postgres

Currently doesn't support GCS, we'd need to add:

• the ConfigMap
• the gcs: ... in the bgbackrest config
• the proper flags to enable or disable the rest

Once the changes are accepted upstream, the cas-postgres-cluster chart can include a dependency on that one