[feature request] Allow specifying namespace of Barman ObjectStore CR

[poehlerflorian]

Currently, the ObjectStore referenced in a Cluster has to be in the same namespace. It would be helpful to be able to specify a namespace where the ObjectStore should be fetched from.

This can have security implications and there should probably a mechanism to enable or disable the access from a different namespace. This could be a flag in the ObjectStore itself.

e.g.:

---
apiVersion: postgresql.cnpg.io/v1
kind: Cluster
metadata:
  name: test-cluster
  namespace: cluster-namespace
spec:
  instances: 3
  plugins:
    - name: barman-cloud.cloudnative-pg.io
      isWALArchiver: true
      parameters:
        barmanObjectName: s3-store
        barmanObjectNamespace: cnpg-system # Add the namespace here
---
apiVersion: barmancloud.cnpg.io/v1
kind: ObjectStore
metadata:
  name: s3-store
  namespace: cnpg-system
spec:
  configuration:
    destinationPath: <destination>
    endpointURL: <endpoint>
    s3Credentials: <s3Credentials>
    # Configure namespace access
    allowAccessFromNamespace: true
    # Maybe allow explicitly specifying the namespace(s):
    allowAccessFromNamespaces:
      - cluster-namespace