Helm chart: database credentials cannot be loaded from existing secret

### Bug description

The structured database credentials introduced in https://github.com/apache/superset/pull/41777 only allow for static values, and have no mechanism to load the password value from a secret. This breaks any workflow that only places database credentials as a secret only, such as CNPG or AWS RDS cluster integration.

This also was a problem with the previous credentials under supersetNode, but I have only encountered this now as I switched to an external database with the Superset V6 upgrade.

I have partially worked around this by specifying an additional `envFromSecrets:` entry, which worked for the application container but is not passed in to the init `wait-for-postgres` job and that still fails in a loop with the stub password entry in the values.

### Screenshots/recordings

_No response_

### Superset version

6.1.0

### Python version

Not applicable

### Node version

Not applicable

### Browser

Not applicable

### Additional context

This is using the latest released version of the chart, 0.22.4.

### Checklist

- [x] I have searched Superset docs and Slack and didn't find a solution to my problem.
- [x] I have searched the GitHub issue tracker and didn't find a similar bug report.
- [x] I have checked Superset's logs for errors and if I found a relevant Python stacktrace, I included it here as text in the "additional context" section.

Start by tracing the Helm chart's structured database credentials and envFromSecrets handling, including the init wait-for-postgres job. Check the rendered configuration for both the application container and the init job. Done means a secret-only database password works for both without the stub password causing the job to loop.

Written by the indexing model from the issue text.