Add a `cncluster psql`

[martinflorian-da]

It is becoming more frequent that we need to open psql shells to databases on clusters.

The steps are roughly:

• inspect the app/Canton deployment that uses the databases, copy IP and database name from there; example kubectl describe pod -n sv-8 -l app=global-domain-6-sequencer | grep -i psql
• inspect the k8s secret that said deployment uses for the postgres password; example kubectl get secret -n sv-8 sequencer-6-pg-secrets -o jsonpath={.data.postgresPassword} | base64 -d
• open a cncluster debug_shell and psql from there
• (optional) do SET search_path <target_schema> for a nicer UX

We could automate this process via adding a nice cncluster command.