Support for multiple users

[kagkarlsson]

Currently the postgres-module creates one "main" user.
It should be possible to create multiple users and have credentials similarly exported.

Use-cases might be

• Multiple application users
• Custom readonly user (which actually would be a good default for all databases)
• replication user for setting up logical replication manually
• Custom user for third-party tooling such as PgHero og postgres prometheus-exporter

Additionally, would it be possible to store the postgres-user password somewhere?

[Glenn-Terjesen]

• [V] would it be possible to store the postgres-user password somewhere