[Feature Request] Delete a database and a user in a running cluster

I would like to be able to delete a database and a user inside a running cluster from the manifest.

• Which image of the operator are you using? registry.opensource.zalan.do/acid/postgres-operator:v1.5.0
• Where do you run it - cloud or metal? Kubernetes or OpenShift? Bare Metal K8s
• Are you running Postgres Operator in production? yes
• Type of issue? feature request

Details

I saw that in database.go there are several constants with the SQL command for each one, it would be interesting to have something like this:

const (
    ...
    deleteDatabaseSQL = `DROP DATABASE "%s"`
    deleteUserSQL =  `DROP USER "%s"`
    ...
)

• In the kubectl pg command, I would like to get a command like delete-db and delete-user instead of deleting the entire cluster

Start with pkg/cluster/database.go, especially the existing SQL command constants, then trace the kubectl pg command entry points. Determine how manifest-driven database and user deletion should be represented and how delete-db and delete-user would operate on a running cluster. Done means the requested operations are supported without deleting the entire cluster and are covered by appropriate verification.