[feature request] add postgres-instance provisioner based on postgres provisioner

[astromechza]

In some cases, the workload expects to receive an admin user credentials for a database host and the app will create it's own database within the instance. We usually call this an -instance resource and it has the same outputs except that it provides the root user and password and without the database/name output.

This should be based on the same shared state as the existing postgres provisioner.

[mathieu-benoit]

Good example with Backstage indeed, thanks for capturing this!

[mathieu-benoit]

@astromechza, as a side note here, in the score-k8s's default provisioner, the implementation is different. I'm wondering in which direction we should go though:

1. Like current score-k8s (i.e. allowing the postgres to create databases)?
2. Or like recommended in this current issue by adding a new postgres-instance but them have this consistent in score-k8s for consistency?

Also, quid of mysql, mssql, etc. where we should go with this approach too for consistency?