As a applicationplatform developer
I want to create and manage postgres service users through the API
so that I can provision a database and associated service user via the API

This would allow us to create a database and a service user for an application, and grant privileges for only that database to that service user and forward the credentials to the application, which would then be able to connect and start using the database directly.

We would avoid having to either share credentials for the avnadmin user, or having to write a separate tool to connect using avnadmin to grant the relevant privileges to the service user.