read-only replicas

It would be nice to be able to create read-only replicas from the operator.

I imagine there would be a "PrimaryClusterId" field in the DatabaseClusterSpec which would, during creation, create a replica.

If this field was deleted down the line, the database could promote itself to a primary.

alternatively, there could be a "DatabaseReplicaCluster". This would involve adding a field like "ReplicaClusterId" in "DatabaseClusterSpec" which would facilitate the promotion & deletion of the old replica.