Add a status report entry and logic to ensure the billwerk API credentials are present

Problem/Motivation

Add a status report entry and logic to ensure the Billwerk API connection is properly configured.
For example "User Sync" is already possible without having connection credentials set.

Unsure, if this should be done in Subman or in the implementations like Billwerk, but these functions should never be triggered without a proper connection.

Perhaps it would even make sense to add a settings page with a "Test connection" button, describing where and how to set the API credentials.

Due to security reasons we shouldn't add a UI that exposes the credentials, but a UI to check it's properly configured.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes