Multiple different products use SQL databases for their metadata. Configuration is not unified, and for Druid, there is a database password in plaintext in the CRD.

Implementation work

We came up with ADR029: Standardize database connections to have a unified way of specifying these database connections. Now it is about actually implementing this. For each product, the CRD needs to be changed, this is a breaking change.

### Adapt product operators
- [ ] Airflow
- [ ] Superset
- [ ] Druid
- [ ] Hive

Documentation

Then, how to configure the database should be documented. This should include a concept level page about the general configuration, which will explain the stuff that we came up with in the ADR029. This page can then be referenced in the operator docs.

The individual operator usage guides should give advice for an SQL production setup. Currently, we do not have that. The only docs we have in this regard are the getting started guides.

### Documentation
- [ ] Write concept page
- [ ] Write Airflow docs
- [ ] Write Superset docs
- [ ] https://github.com/stackabletech/druid-operator/issues/462
- [ ] Write Hive Docs

Follow-up work

### Tasks
- [ ] Adapt all the demos. This is usually done just at release time.