Move Database config to a secret

[fhennig]

As a user I'd like to be able to configure the database credentials using secrets

This is done when

• connection credentials are not part of the CR anymore but instead are split out to a secret
• integration tests, examples and documentation have been updated

[fhennig]

s3 secret reference into a complex enum and think about other storage solutions

I think I just copied this from the Google doc when I made the ticket.

I think what is mean here is that you can just have a key backendStorage and then multiple options, this can be implemented with different enum variants. But not for now.

[lfrancke]

When refining this please note that it might end up as a bigger task (i.e. other operators might have the same issue and we might want an ADR / consistent CRD).
As such it could be that this ticket needs to be changed to a research task but I'm not entirely sure so I'll leave it like this for now.

[sbernauer]

Just wanted to mention, that the acceptance criteria of this ticket as well as the description originally meant to only fix the bug we currently have. This should be rather easy and doable in less than a day.
We have a ticket for a generic reusable struct: stackabletech/issues#238
It was my fault to not link that in July during creation...